Horses need the freedom to use their head and neck when jumping down a drop. This means you have to slip the reins (the technical term for letting the reins slide through your hands!
If you don't, then when they get to the lip of the drop with you (the rider!) holding onto their mouth, they are forced to be hollow and jump outward instead of lowering their neck and plopping neatly down.
It's human nature to try to hold on for dear life (it's our hands that set up apart after all!), but it is necessary to let go!
So, how to be brave enough to slip the reins?
Slow down early and thoroughly.
Let the reins slip through your hands as the horse lowers his neck. This allows the horse to plop neatly down.
Remind yourself that it's your legs that keep you on the horse, not your butt!
Check out the difference between the first and second videos of Morgan Mindrebo practicing a drop with Alex. Note how stiffly Alex jumps in the first video when he does not have the freedom to lower his neck and how softly he jumps when he does in the second video!
When you land with a long rein and need to steer to a chevron (like Morgan did in the second video), you have to think quickly! Do you have time to shorten your reins, or is it more efficient to widen and raise your hands to steer on long reins?
Practice shortening your reins AND steering with your legs at home to a wide jump! Don't let the first time you try it be at a horse show!