You absolutely do not need any kind of special rein. It is better to start on the ground, and build up to the saddle. To teach my horses to back on the ground, I face the horse, tap the chest-shoulder area with the lead rope and say b-a-a-a-a-a-a-c-k in a low, commanding tone, stepping forward and way crowding the horse until they take a step back. If I am in the saddle, I sit deeply, close my leg, and cluck or say in a deep, commanding voice, b-a-a-a-a-a-a-a-c-k while half-haulting on the reins until they take a step. Always go for one step first, then once you know they've got that concept down, build up until they back as long as you want, moving their diagnal legs in pairs (i.e., front right and left hind move together, then front left and right hind, etc) as they go back.