Starting position: Partners face one another and both move the right leg forward. B bends his arm with fist facing up. A holds B as if they’ll play hand wrestling with her right hand and with her left palm she supports B’s elbow from below.

Performance: B begins to move his fist to his shoulder overcoming A’s resistance. On the way back to the starting position the muscles should be relaxed. After several repetitions B changes his hand and after that partners change roles. 
You can see another variant of the exercise on the photo to the left.