Awesome! I'm sure you will love the bit. Yes, you are combing the reins from the withers up. If you use leather reins they won't slide as smoothly through your hands so you need to make sure that you aren't getting little tiny jerks while combing (especially if you have braided reins). I use rope reins made out of Samson yachting rope and the feel is awesome. I've done this exercise with several horses using leather reins and while not ideal it still works.
By don't block him, I mean that when he goes to put his head down you don't hold onto the reins so that he jerks himself in the mouth. You want to allow the reins to slide through your hands when he goes to put his head down. Once he has the general idea and is kind of consistant, when he puts his head down instead of stopping the combing action altogether, keep combing the rein you still have left. It may not be much, but that's okay! Keep that steady, rhythmic feel.
One big key here is NO FLOPPY REINS! Lol. That was my biggest problem. I was, in a way, afraid to put much contact on my horse's mouth because of his confidence issues, so when he would invert my reins would get floppy, thus defeating the purpose of the exercise. So the higher he puts his head, the more drag you put in the reins. And believe me, I had to put A LOT of drag in my reins with my horse! But he had some deeply imbedded confidence issues with the bit, so your case may not be as extreme.
Good luck!! Let me know how it goes! :)