Sit upright on the floor with your partner behind you. Raise one arm straight up, and flex the elbow, attempting to touch your hand to your back. Your parner should hold your elbow and wrist. This will be your starting position.
Attempt to extend the arm straight into the air as your partner prevents you from doing actually doing so.
After 10-20 seconds, relax the arm and allow your partner to further stretch the tricep by applying gentle pressure to the wrist. Hold for 10-20 seconds, and then switch sides.