The slider cap is used with the L.E. rear wings, and allows the wing to slide fore & aft about 3/4" of an inch. This adjustmanet allows the rider to tailor the "quickness" of the foil to his or her liking. Moving the rear wing all the way back will slow the foil down and make it more stable. Moving the rear wing forward will speed the foil up, make it more responsive and aggressive.