A fighting move. Applying pressure with your thumb underneath and behind the jaw. Not effective under the chin. You really want to get your thumb in behind the jawbone and hold the pressure as long as neccesary. Excellent if someone has you in a hold. Less permanently damaging than gouging their eye out.
"Mike's in a choke hold. It doesn't look like he can get out of this! But wait! He just used the fish hook on his attacker! He's out!