The half pass should be asked for with a combination of aids: outside rein and inside leg. The outside leg comes back and is on the horse’s side, but it’s just a cue. The combination of outside rein and inside leg make the quality. So the first thing the horse has to know is that your inside leg is serious, and then he must move off of it enough that he fills the outside rein and has bend through the body. In a good half pass, the inside rein is completely unnecessary.
That drooping inside rein is important because it gives freedom to the shoulders. A tight inside rein stifles the shoulders from being expressive. Some think that moving more sideways creates more expression. In fact, it’s the opposite. Expression and reach create a more sideways half pass. That’s why horses like Ravel half pass so beautifully; they are so loose in their front ends that their half passes are extraordinary.
When riding half pass, it’s hard to use your inside leg because it is counter-intuitive to use the leg you want the horse to move into. I find it’s also hard to put the inside leg on once you’ve started the half pass without it. So what I have my students do is to start their half passes from a leg yield. Try this: To do a half pass left, turn down the third quarterline and leg yield right toward the track off your inside (left) leg. When the horse is honestly moving away from your leg, begin half pass left, focusing on keeping your left leg active so he retains the body bend and reach. As soon as the horse bulges against your inside leg or loses the bend (or you stop being able to make your left leg work), leg yield right again and only resume half passing when he’s honestly off your inside leg. Don’t forget to do the exercise in the opposite direction, too.
This exercise is as much for the rider as it is for the horse. Both you and your horse need to believe the inside leg is serious. Often, because it’s counter-intuitive, a rider tries to use the inside leg in a half pass and can’t. But if you want to do a leg yield, it goes on straight away. Both you and he need to accept that the inside leg means move over. That’s how we obtain bend through the body; the inside leg pushes the barrel over.
Half pass–leg yield–half pass as many times as you need to. Do as many strides of half pass as are good and no more. In the beginning, you may do more strides of leg yield than half pass; if in doubt that your inside leg is effective, leg yield to make sure. Your half pass to leg yield and back should be, as Schumacher says, “like butter.” Of course, The goal is to have to do fewer and fewer strides of leg yield but even Grand Prix riders do this. It’s just invisible because it all happens in half a stride.