the yolk that slips into the tranmission. it has inner splines. if you want to know why and how they're used or modified, i've explained a little below about how to get rid of them:
heavy duty rock crawlers use slip yoke eliminators because often times, the u-joints, differentials, and half-shafts they've upgraded to will take the twisting and larger tire weights easily, but the splines on the inside of the yolk (that turn with and are part of the transmission output shaft), can strip right off, therefore kits are made to eliminate the spline-drive and enhance the strength by allowing the transmission to terminate the output shaft using a u-joint itself.
that allows better driveline angle changes too, because the u joint is now part of the housing, and not pushing up on a bend in the driveshaft on some of the more extreme driveline angles, which would bend the yolk within the housing in certain situations, also due to the u-joints orientation.
if you make the u joint the end of the transmission, it now becomes the pivot point.
just incase you're wondering. =)