Why is the stop so important before the rollback?

The rollback is entirely dependent on the quality of the stop that initiates it, because the stop determines the horse's body position, balance, and physical state at the moment the turn begins — and the turn can only be as good as the foundation it starts from. A horse that stops straight and balanced, with its hindquarters driven under its body and its back flat, arrives at the rollback in the correct physical position to sweep its front end over the hind end cleanly. A horse that stops crooked, braced, or strung out arrives at the turn in a compromised position from which a correct rollback is physically difficult or impossible. Crookedness in the stop means one shoulder is loaded more heavily than the other, which causes the turn to either fall through the low shoulder or lock against the high one rather than sweeping freely in the intended direction. A horse that stops braced — with its jaw, poll, or back stiff from the contact or the speed — arrives at the turn with physical tension that restricts the free shoulder movement the rollback requires. A horse that is strung out — stopping on the forehand without the hindquarters driven under the body — has no anchor from which to turn, and the rollback will travel forward rather than rotating in place over the hind end. In training and in competition, riders who focus on the turn itself and accept a compromised stop as acceptable are working backwards: the stop is the foundation, the turn is the result. Time spent on the quality, straightness, and softness of the stop always improves the rollback more directly than time spent specifically drilling the turn on top of a poor stop foundation. Every rollback problem traces back to either the stop that preceded it or the departure that followed it — rarely to the turn in the middle.