How do I teach my horse to rate at the barrels without losing momentum?

Rate at the barrel is the controlled deceleration that allows the horse to turn efficiently — it is not a full stop and should not feel like one. A horse that loses too much momentum through the rate loses time on every barrel; a horse that fails to rate will blow past the turn entirely. Teaching rate begins with teaching the horse where the turn is: repetition at slow speeds, approaching the barrel correctly, and allowing the horse to find its own footing in the turn without interference. As speed increases, introduce a soft half-halt a horse-length before the turn point — a brief closing of the fingers and a slight weight shift back that signals the turn is coming without dramatically slowing the horse. The horse should collect its stride, not slam on the brakes. Work on approach lines: a good straight approach allows the horse to engage its hindquarters under its body before the turn, which is what produces a tight, fast arc rather than a wide drifting turn. Horses that blow past barrels often do so because they are approaching with their bodies bent away from the barrel rather than tracking straight. Practice approach work at controlled speeds until the horse knows exactly where the turn happens and begins to organize itself without being told — that self-organization is true rate.