As promised, here is some video of Coriander's jaw flexions.
Of course there are multiple ways to do this, I'm using this method because it directly translates to single rein riding mechanics. This is my process: I put up a "tai-chi wall" with the rein, then I steady my hands while putting a little pressure on his mouth, and then I wait for him to give his jaw. When he gives the jaw I click and immediately drop the reins. The release is key, if you don't release the horse will just stiffen up and brace against you.
You can see right at the beginning of the first video where I should have clicked and released but didn't. Coriander certainly noticed, he gets a little grouchy when he knows he did something click-worthy and doesn't get clicked. Then he got totally distracted by the horses across the way- they're what he's staring at so intently. At one point I waited two minutes for him to relax his jaw. Lucky for you I didn't include that video, it's more boring than watching grass grow, but I did want to mention it to prove how patient you need to be sometimes to get a response from your horse.
Notice how his mane flips over to the right around his poll? His neck is crooked, he always carries his head a little to the left, which makes jaw flexions to the right a little more difficult for him. I'm considering getting a chiropractor out to look at him but I want to work on the jaw flexions a little longer before I do that. How cool would it be if they straightened his neck out?