So, you feel behind the highest bone on the top of their head and push behind it with your finger tips. It's a sort of pressure point and makes them put their heads down. It's very nifty! I've found that some horses get grumpy about it, but once they understand that when they lower their heads and you release and give them 'praise' and make it a good place, they lower their heads very willingly. Soon, as soon as you put your fingers in that spot (I like to call it a 'button') they know what you're going to ask and just put their heads down.
It's awesome for horses that are a PAIN to bridle, I did it with my dads massive appendix QH mare.
Here's a quick video of the second or third time I've done this with my big beast, Squiggy.
Head Lowering Trick - YouTube
Anyone else do this trick?