Eccentric shoulder external rotation in sitting with load

Sit at the corner of a table.
Hold a weight in one hand and place your elbow out to your side on the table, slightly in front of your body, elbow bent to a right angle and hand pointing up.
Place a rolled towel or ideally a small ball under your elbow.
Slowly lower the weight towards the floor in the direction of your opposite knee.
Return to the starting position assisting with your other hand.