In the middle of the jump

Don't use the reins, use your leg. Hold both reins steady (even pressure). Wich way does he shift? If he shifts left, you need alot of left leg. If he doesn't respond, use a little bit of LEFT rein and left leg. If you pull your right rein, he will just bulge his body more left. If he is a right shifter, reverse this

If he doesn't have a specific way, just hold both legs and both reins evenly and LOOK UP. It is amazing how much straighter they move when you look up.

A helpfull excersize is to put a pole (on the left if he shifts left, or visa versa) . Place it so it is about two feet from the middle of the jump, perpendicular. This will help you understand where he needs to be

i agree with all of the above

I also agree. You definitely need to use your legs with your reins. If he tends to drift to the right, you need to close your right leg and your left rein. Try jumping some crossrails too. Sometimes it helps because there is a clear distinction as to where the middle really is.

Never use inside rein to direct your horse back to the inside. Always use outside rein.

So if he is drifting to the right *more than likely because your back needs realigning and you are causing the drift* then you use our right leg behind the girth and right rein on his right shoulder to direct him back to the center.

You can pull their face all you want with your inside rein, but that isn't going to cure the issue. You must direct the ribs and the shoulders. Your outside leg directs the ribs, and your outside rein keeps that outside shoulder under your horse, which then in turn gives your horse nowhere to go, but to the left.

You have to block their energy from flowing in the incorrect direction.

So if your horse was veering to the left, left leg and left rein on the shoulder.

I agree...you need to keep the elbow closed (the left elbow if he drifts left, right if he drifts right) ...meaning keep it tightly at your side and a little contact on your rein...then close the same leg hard....dogscatsrodents did a good job explaining...