The usual formula for riding is: Aid + Timing + Intensity. All three factors interact, so you give the right aid at the right time with the right volume.
You may apply the correct aid with the right pressure, but if you do so a little early or late, the horse won’t respond the way you intend. Or you may give the correct aid at the right time, but if it’s a little too loud or soft you again get the wrong response, at least from your point of view. The horse won’t know it’s "wrong," because from his point of view that’s what you asked.
Over-busy hands throw the formula out of whack. We’re used to operating brain-to-hand. That’s what we do when we write or, even more so, when we use a computer all day. We want the horse to turn and we pull on a rein. We want the horse to slow down or stop and we pull on the reins. We want the horse to lower his head and we see-saw the reins.
When you ride brain-to-hand, you’re mostly riding the part of the horse that’s in front of your own eyes, and you’re leaving out the part that’s doing all the work. You also tend to tilt your body forward, which makes the horse lean on your arms. Rein action can slow the shoulders too quickly and cause the haunches to slip out the side, just like what happens when you brake quickly while driving a car with front-wheel drive.
In a snaffle bit, if the reins are the right length, with no looping but no tension, you can turn the horse simply by turning your shoulders. You can halt by closing your legs so that the hind end comes up to meet the front end. The horse will lower his head if you leg-yield slightly so that he softens down across the withers. The horse stays balanced equally over all four legs. He carries you instead of the other way around.
All this sounds like various ways to half-halt. It’s simpler to explain the half-halt as an idea rather than a specific set of aids.
A half-halt will re-balance the horse, help him focus his attention on you and prepare his body for the task that comes next. If your half-halt goes straight to your hand, none of those good things happen because the horse just braces his neck.
When you school, put thought into the other necessary aspects of a hand aid, the timing and intensity. Try a series of downward transitions and each time use 50 percent less hand, until your hand aid whispers instead of shouts. The idea is to focus your brain away from your hands to where your body connects most directly to the part of the horse that’s doing all the work and that’s holding both of you upright. Use the eyes you have in the back of your head, not the ones you have in the front.