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 tricep and wrist. This will be your starting position.
Attempt to pull your upper arm to your side 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 lat by applying gentle pressure to the tricep. Hold for 10-20 seconds, and then switch sides.