Trick training: How do I train my horse to bow?

I thought it would be really cool if I trained my new horse to bow, so like at the end of a showmanship class I could tap his knee or something and have him bow.
I was wondering the best way to go about doing this?
How do I train him to bow?

Yes I want to leanr without having to rig up ropes to drop him, I don't like that idea. I want him to have a choice.
TELL US!!!