Question: Which technique where the roller coaster is allowed to coast through the ride's circuit through the force of gravity is used in Adventure World since 1991?

Context paragraphs:
Lift hill: A lift hill, or chain hill, is an upward-sloping section of track on a roller coaster on which the roller coaster train is mechanically lifted to an elevated point or peak in the track.  Upon reaching the peak, the train is then propelled from the peak by gravity and is usually allowed to coast throughout the rest of the roller coaster ride's circuit on its own momentum, including most or all of the remaining uphill sections.  The initial upward-sloping section of a roller coaster track is usually a lift hill, as the train typically begins a ride with little speed, though some coasters have raised stations that permit an initial drop without a lift hill.  Although uncommon, some tracks also contain multiple lift hills.

Doo Wopper: Doo Wopper is a steel roller coaster on the Surfside Pier of Morey's Piers amusement park.  The attraction appeared along with Rollies Coaster, The Storm, and Shark Bite, one of which are defunct.  The Doo Wopper was relocated from the Adventure Pier to Surfside Pier where it replaced the RC-48.  A S&S Screaming Swing took the Doo Wopper's former space on Adventure Pier.  The Doo Wooper is a wild mouse type of roller coaster.  This ride has undulating hills and is a modern version of a classic wild mouse coaster.  The ride costs five tickets to ride.  Morey's Piers has rated this ride as a moderate type of roller coaster.

Temple of the Night Hawk: Temple of the Night Hawk is an enclosed roller coaster located at Phantasialand.  Founded in 1988 after 18 months of construction, the ride was originally called ‘'Space Center'’.  The ride led passengers past models of rockets and emulated asteroids and the darkened hall was dimly lit with thousands of tiny spots of light to resemble stars.  Due to the construction of the neighbouring Wurze Town in 2001, the roller coaster was redeveloped with a fantasy jungle theme and was renamed as the "Temple of the Night Hawk".  The roller coaster is now completely dark.  Until 2006 there were some green moving lights and strobe lights in the first lifthill, still present today, but not operational.  In 2008 the Lights on the coaster trains were turned off or broke down and have never been repaired since then.  Till 2010 there was a green laser projection with the outlines of a flying hawk and until June 2012, there were some red strobe-lights at the final brakerun.  In 2011, the park installed new strobe lights in the first lifthill, which broke down after about two months, and were never repaired since then.  The ride has four trains and one spare train for repairs each with seven cars.  Each car has two rows of seats each holding two people.  During the four-minute ride time the trains are lifted by three lift hill chains, 1.174 metres of track.  The roller coaster is therefore one of the longest indoor roller coasters in the World, slightly beaten by "Mindbender" in Galaxyland Amusement Park, Canada.  The hall in which the track is located is around 120,000m³ in size and rests on 180 concrete pillars which are 12 metres deep in the ground.  In the basement of the hall is the ‘'Hollywood Tour’' darkride.  The total investment for the ride was around 15 million DM.

Millennium Force: Millennium Force is a steel roller coaster built by Intamin at Cedar Point amusement park in Sandusky, Ohio, United States.  It was the fourteenth roller coaster to be built at the park since Blue Streak opened in 1964.  Upon completion in 2000, Millennium Force broke six world records and was the world's first Giga Coaster, a roller coaster that exceeds 300 ft in height and completes a full circuit.  It was briefly the tallest and fastest in the world until Steel Dragon 2000 opened later the same year.  The ride is also the third-longest roller coaster in North America after The Beast at Kings Island and Fury 325 at Carowinds.  It was the first roller coaster to use a cable lift system rather than a traditional chain lift.  The coaster has a 310 ft , 45-degree lift hill with a 300 ft drop and features two tunnels, three overbanked turns, and four hills.  It has a top speed of 93 mph .

Turbo Mountain: Turbo Mountain was a steel roller coaster at Adventure World in Perth, Western Australia, Australia.  The ride was closed and removed in 2009 to make room for a HUSS Shot'N Drop tower named Freefall.  The ride was originally located in Luna Park Sydney as a standard Schwarzkopf Jet Star 2 bought second hand.  When the ride was moved to "Adventure World" in 1991, the ride's lift hill was modified from a spiral lift hill to a standard chain lift hill due to maintenance.

Green Lantern Coaster: Green Lantern Coaster is a steel roller coaster at Warner Bros.  Movie World on the Gold Coast, Queensland, Australia.  The ride is themed after DC Comics' Green Lantern and is located within the park's DC Comics superhero hub.  The ride is an El Loco roller coaster manufactured by S&S Worldwide, characterised by a tight circuit featuring a beyond-vertical drop and an outward banked turn.  It holds the record for the steepest drop of any roller coaster in the Southern Hemisphere, and the second steepest in the world.  "Green Lantern Coaster" officially opened on 23 December 2011.

Leviathan (roller coaster): Leviathan is a steel roller coaster in the Medieval Faire section of the park at Canada's Wonderland in Vaughan, Ontario, Canada.  It is the first roller coaster designed by Swiss firm Bolliger & Mabillard to exceed a height of 300 ft , putting it in a class of roller coasters commonly referred to as "giga".  At 5486 ft long, 306 ft tall, and with a top speed of 92 mph , Leviathan is the tallest and fastest roller coaster in Canada.  s of 2015 , Leviathan is ranked as the eighth-tallest, and the eighth-fastest roller coaster in the world, and the third-tallest traditional lift-style coaster in the world.  It was the 16th roller coaster to be built at Canada's Wonderland, and the ride's track was completed in February 2012, with the first test run being completed on 15 March 2012.  The coaster opened to season pass holders on 27 April 2012, and to the general public on 6 May 2012.

Hurricane: Category 5: Hurricane: Category 5 was a Custom Coasters International wooden roller coaster located at the Myrtle Beach Pavilion.  It replaced the Corkscrew roller coaster which existed since the late 1970s.  The Pavilion unveiled their multimillion-dollar coaster May 6, 2000.  During operation, Hurricane held the record for being the tallest, fastest, and longest wooden roller coaster in South Carolina.  The ride closed with the Pavilion on September 30, 2006.  Although Burroughs & Chapin attempted to sell the ride along with the Haunted Hotel, Log Flume, Treasure Hunt, and a few other rides, the ride was deemed too expensive a task to dismantle and relocate, and was ultimately demolished in March 2007.  The only part of the ride not demolished were the two Gerstlauer trains used on the ride.  These trains were shipped to Kings Island, an amusement park in Mason, Ohio.  They were then repainted and installed on Son of Beast, which was at the time the world's tallest and fastest wooden roller coaster.  Son of Beast was later demolished on November 20, 2012, following an incident that occurred in 2009.

White Cyclone: White Cyclone (ホワイトサイクロン , Howaito Saikuron ) is a wooden roller coaster at Nagashima Spa Land in Mie Prefecture, Japan.  At 1700 m in length, White Cyclone is the third longest wooden roller coaster in the world, and is the longest wooden roller coaster outside of the United States.  Despite its length, White Cyclone is still considerably shorter than the 2479 m Steel Dragon 2000, the world's longest steel roller coaster, which is also at Nagashima Spa Land.  In addition to being the third longest wooden roller coaster, White Cyclone is the seventh tallest wooden roller coaster in the world and the fourth tallest wooden roller coaster outside the United States.  A single ride on the White Cyclone costs ¥1,000 (approximately $9 USD), and the ride is restricted to those individuals above 1.3 m in height; and those individuals under 54 years of age.

Top Thrill Dragster: Top Thrill Dragster is a steel accelerator roller coaster built by Intamin at Cedar Point in Sandusky, Ohio, United States.  It was the sixteenth roller coaster built at the park since the Blue Streak in 1964.  When built in 2003, it was the first full circuit roller coaster to exceed 400 ft in height, and was the tallest roller coaster in the world, before being surpassed by Kingda Ka at Six Flags Great Adventure in May 2005.  Top Thrill Dragster, along with Kingda Ka, are the only strata coasters in existence.  It was the second hydraulically launched roller coaster built by Intamin, following "Xcelerator" at Knott's Berry Farm.  The tagline for Top Thrill Dragster is "Race for the Sky".

Supporting paragraphs: Turbo Mountain Lift hill
Answer: chain lift hill