Sentence:  [page 77] Riders should adapt their own style when doing <WSD>Supermans</WSD>, and shouldn't get hung up trying to do it the same way that someone else does. ... [page 79] An extension of the <WSD>Superman</WSD> is a trick called the Superman Seat Grab. Even more impressive than the <WSD>Superman</WSD> itself, this trick was again brought over from the BMX world, ... During the Superman Seat Grab, ... the rider lets go of the left side of the bars and grabs either a cutout in the rear number plate or a grab handle that has been previously installed. With his body moved toward the back of the bike, the rider is able to extend the bike way out in front of himself, almost floating behind the machine. ... Whichever way they are done, the <WSD>Superman</WSD> and its variations are some of the most impressive and difficult tricks out there. 
Meaning of the word marked with <WSD> tags:  A stunt in which the rider releases both hands from the handlebars in mid-air.