The Arm-Bar is the absolute truth.

There is a modified one that isn't shown... For the vertically challenged, like me, going up against someone that is tall- keep his hand/fist under your armpit, put your forearm under his elbow gripping your other forearm, lean back slightly, but push your forearm up in the air to apply pressure to his elbow.

Im not great at explaining, but for those that know, know..

And for those that feel the need to practice on a smaller person in the family, DONT rush the process cuz you will dislocate something.