Some people don’t recommend putting your hands in your mouth because the force of the jaw closing could injure your fingers……but
This works for me: right after you notice the pop feeling that your jaw has locked, relax…it will come undone! Take your two pointer fingers and stick them in your mouth and grab your farthest back teeth (your molars or whatever), one side with each hand. Wrap your thumbs around your jawbone. Press down firmly with your pointer fingers sort of down and back while relaxing your jaw. Hopefully it will reset itself. The quicker you set it the better.
Also see your dentist about it. More Info
I’m not a medical person of any sorts, this just works for me. Once my mouth got stuck open for a half hour. That really sucks. If you hit this page I hope it helps a little.
Feel free to call this the Ellerbeck Maneuver if you so desire Not as life saving as the Heimlich but we all do our best!