How do you develop adjustability in the canter for hunter jumper horses?

Developing adjustability in the canter — the ability to lengthen and shorten the stride within a consistent rhythm and balance without breaking to trot or rushing — is one of the most important training goals for hunter jumper horses because adjustability is what allows the horse to arrive at fences on correct distances regardless of where it is in the approach. A horse with genuine adjustability can compress its stride to meet a close distance or extend its stride to meet a long distance while maintaining the same rhythm and balance, making course navigation a matter of strategic pace adjustment rather than a gamble on whatever distance the unadjusted canter produces. The primary exercises for developing canter adjustability are transitions within the canter — collecting the canter to a shorter, more balanced stride and then releasing forward to a longer, more ground-covering stride within the same gait without breaking. The quality of these within-canter transitions — whether they happen promptly when asked, whether the rhythm is maintained through the transition, and whether the horse returns to the original pace after the adjustment — indicates how well the adjustability training is progressing. The half-halt is the primary tool: a half-halt that effectively shifts weight to the hindquarters and reorganizes the canter's balance is the mechanism through which the horse shortens its stride, and the release after the half-halt is the mechanism through which it extends again. Cavaletti and ground poles set at varying distances — sometimes at a comfortable normal canter stride length, sometimes at a shorter distance requiring compression, sometimes at a longer distance requiring extension — develop the horse's feel for adjusting its stride to arrive at a specific point and reward correct adjustment with a comfortable departure over the pole. The adjustability developed through flatwork work is tested directly over fences: a horse that can compress three strides into a short four in a related distance and lengthen four strides to a forward four in a different line has developed the specific adjustability that course riding requires.