Kneel down gripping a corner in each hand with your partner gripping the other two corners and standing behind you (with towel over one shoulder switching sides per rep or just keep it centered above your head).

You then extend your arms out while he pulls, and then when you’re extended you bring your arms back slowly as your partner continues to pull providing negative resistance. It’s the best tricep exercise I’ve done. 3 sets, 10 reps, as hard as you can do it.