I'm not sure (because I ride dressage, and have only been on a reiner once, lol), but they may be bumping to ask the horse to lift the back, which will make the head go down.
A hollow horse won't be able to lower it's head, so you need to get the back up and round. I wouldn't be surprised if there was a spot to give a little rub (w/ a heel or spur) to say "lift it up." We do a similar exercise off the horse w/ hoof picks (insteadof spurs) to get them to practice using those muscles.
So, for now, if you have any cue that means "be round - lift you back" it will probably work. Just be sure your seat isn't saying "go" (forward or back).
Edit because "hell" and "heel" are not the same