pucker
v.— «“No matter how many times you do it, you still pucker once you get over the wire,” says one of the helicopter’s pilots, Chief Warrant Officer Lance Duensing.» —“The Doctors of War” by Bob Drury Men’s Health Jan. 30, 2006. (source: Double-Tongued Dictionary)
Pucker – refers to the action taken by the sphincter or “butt hole” when the human body is experiencing great stress or fear. Commonly referred to as “the pucker factor”
Yep, that’s right. No mystery there.