Instead of dropping g your hands down, causing your arms to straighten, think of just pushing your arms forward (while keeping your shoulders back), then drive your horse into the contact with your leg.
Normally (in my experience) if you just let out like a centimeter of rein then your horse should want to stretch down. This will help him know what you want and you to still have a hold on your reins. It will prevent giving your arms a
By bend down do you mean supple? If so then I would I steady take your elbow back to your hip to create muscle memory