the horse will do things the first time almost by chance or accident. He gets rewarded for that , the next time he is 'cued' to canter depart, he may try all kinds of things, such as speed up, back up, throw his head, whatnot, but the rider continues to cue until, almost by accident, the horse canters, "cue " ceases. It doesn't take long for a hrose to associate his correct action with the release of the cue. The cue, in and of itself, is relatively meaningless, initially. However, the body position of the rider, in cueing for a right lead, or a left lead canter, supports and mirrors the position the horse will take as he strikes off. If the rider advances the inside hip, this will mirror the way the horse will advance his own inside hip. The rider's shoulders should mirror the horse's desired postion of HIS shoulders, which will be a bit different than the hips.
It's amazing how accurately the hrose will pick up on and follow our body position.