They work quite well.
Sorry...
One way:
Stand in a power rack.
Position the hooks at about waist height and set a barbell in them.
Grip the bar with both hands about shoulder width apart.
Stand with your body almost against the bar and, with one foot, take a nice sized step back.
Now move your other foot to meet it.
Feet together.
Bend a bit at the hips.
Tuck your chin.
Bending the arms at the elbows, slowly lower your head under the bar until you get a decent stretch. Keep your elbows pointing down and in with constant tension on the triceps.
The rest of the body remains fairly rigid.You’ve just done the negative.
Now reverse the above actions and push yourself back to the starting position by basically doing a triceps extension.
Really, that’s all this is, but instead of moving the bar away from your body, you’re moving your body away from the bar.
You’ll probably have to experiment a bit with minor adjustments to bar, feet, and hand positioning to get the right "feeling" for you.
Judging from your pics, I’m confident you’ll know by feel when the movement is right.
So give it a shot and report back.