Its just a big, crazy yet important tool for doing major engine work. Pulling Cranks, Cams, etc.
The joke comes from Dave Hough's "Proficient Motorcycling" where he tells a story of an overloaded motorcycle. One of his friends had his Motor Company bike so overloaded he wouldn't let his wife take more shoes. The cause behind the overweight condition was FULL set of snap-ons, including a Jaw Puller (which most bike mechanics would only have around for pretty dire circumstances, its a pretty heavy gadget.)
So now, when I ride with my Motor Company bretheren, I needle them a bit about:
A. Having almost 150 pounds more cargo capacity than a Road King and,
B. If they need me to bring along a jaw puller for their bike I could accomadate them.