Question: How does tapping the 57 label on the Heinz bottle help with getting ketchup out?
About the article "Ketchup", section "Viscosity".

Candidate passages:
Tomato ketchup has an additive, usually xanthan gum, which gives the condiment a pseudoplastic or "shear thinning" property.
This increases the viscosity of the ketchup considerably with a relatively small amount added - usually 0.5%.
- which can make it difficult to pour from a container.
However, the shear thinning property of the gum ensures that when a force is applied to the ketchup it will lower the viscosity enabling the sauce to flow.
A common method to getting ketchup out of the bottle involves inverting the bottle and shaking it or hitting the bottom with the heel of the hand, which causes the ketchup to flow rapidly.
A technique involves inverting the bottle and forcefully tapping its upper neck with two fingers (index and middle finger together).
Specifically, with a Heinz ketchup bottle, one taps the 57 circle on the neck.
This helps the ketchup flow by applying the correct shearing force.
These techniques work because of how pseudoplastic fluids behave: their viscosity (resistance to flow) decreases with increasing shear rate.
The faster the ketchup is sheared (by shaking or tapping the bottle), the more fluid it becomes.
After the shear is removed the ketchup thickens to its original viscosity.
The relatively high viscosity and pseudoplastic nature of ketchup can make pouring it from a glass bottle somewhat difficult and unpredictable.
According to the Heinz trivia website ketchup exits the bottle at a speed of .
The website also notes that hitting the "57" mark on a glass Heinz ketchup bottle makes the ketchup pour out more quickly and easily.
The "New York Times" also notes that Heinz placed the "57" mark on that particular spot of the bottle as the best place for consumers to hit for the ketchup to pour smoothly.
The Heinz website claims that only 11% of consumers know this fact.
However, while the number "57" is used for the function of pouring, the number "57" resembles all the different types of products that the company offered.
The company actually manufactured 60 products when Henry Heinz introduced the number "57" on his ketchup bottles, Henry thought the number "57" was lucky and continues to use it today. 
The relatively high viscosity and pseudoplastic nature of ketchup can make pouring it from a glass bottle somewhat difficult and unpredictable, and several urban legends surrounding this phenomenon have arisen.
According to one popular folk remedy, repeatedly hitting the "57" mark on a glass Heinz ketchup bottle makes the ketchup pour out more quickly and easily.
The "New York Times" claims this is a matter of intentional design, with Heinz having placed the "57" mark on that particular spot of the bottle as a target for consumers to hit.
Whether or not Heinz intended the 57 as a target is unconfirmed; however, the effectiveness of this method of ketchup disbursement has been empirically confirmed as universally effective.
Simply hit the 57 firmly with the palm of your hand in order to free the flow of ketchup. 
The H. J. Heinz Company, also known as the Heinz Company and commonly known as Heinz and famous for its "57 Varieties" slogan and its ketchup, is a United States food processing company with world headquarters in Pittsburgh, Pennsylvania.
The H. J. Heinz Company manufactures thousands of food products in plants on six continents, and markets these products in more than 200 countries and territories.
The company claims to have 150 number-one or number two-brands worldwide.
Heinz ranked first in ketchup in the US with a market share in excess of 50%; Ore-Ida label held more than 50% of the frozen-potato sector.
On February 14, 2013, Heinz agreed to be purchased by Berkshire Hathaway and 3G Capital for $23 Billion. 
For the year 2000, the Heinz company decided to try something new for its pickle pin.
For the first time, an entirely different pin was produced.
Rather than a pickle, the new pin was shaped like a red Heinz ketchup bottle, complete with painted white cap and keystone-shaped label.
The first batch of ketchup pins made also feature a printed "2000" to denote its commemorative status.
A total of one million ketchup pins were produced, which is roughly the equivalent of a year's worth of pickle pins.