Stand with your hands by your sides.
When ready, lift your arm towards the ceiling as high up as comfortable.
Your partner will pass you a weight at the top of the movement.
Slowly lower the weight down to a supporting surface by bending your elbow.
Then lift your arm back to the starting position without the weight, and repeat.
As a progression, use a lighter weight when returning to the start position in the upward phase of the exercise.
To increase the difficulty of the exercise, you can increase the weight as advised, but do not increase the speed of the exercise.