A true splitter has backspin, or even offset backspin kinda like a 2 seamer. The balls that knuckle or tumble are actually forkballs.
Forkballs are generally held deeper in the hand and they can have some crazy movement. Splitters tend to look more like fastballs, then drop and/or fade at the plate.
Against really advanced hitters (beyond college), forkballs can sometimes be easy to pick up while splits are more deceptive.
Clemens and Schilling throw true splitters--there are not too many guys throwing forkballs any more in the big leagues.