The armbar is basically a hyper extension of the elbow joint.
Hold the arm at the wrist, hug it tight so it kind-of becomes part of your upper body, then extend the hips to hyper-extend the elbow.
A straight arm bar is commonly executed from mount and guard.
Arm bar from guard:
Arm Bar from Mount:
The arm bar from mount is not seen very often in MMA (exception being Rhonda Rousey) as it is quite a ‘sacrificial’ technique. You give up position if you don’t pull it off.
Defending arm bar from Guard
You want to drive in and stack, don’t pull out!
Arm Bar from Mount Defense