This move looks simple, but you’ll want to move with slow deliberation the whole time. By hinging at your hips and possibly recruiting support from a bench or weight rack, you can maximize focus on your triceps.
Keep your upper arm locked into place while extending your arm all the way back behind you to lockout. Squeeze at the top and slowly bend your elbow to come back to the starting position. Your upper arm can work hard to keep itself in place, but it shouldn’t really be moving.