I recommend them as a finishing movement for back so that you are as warm as you can possibly be and the reps should be relatively high, in my opinion. I also don't recommend doing them across a bench but rather lying on the bench just as you would do a tricep extension with your head hanging slightly over the end of the bench. The reason for this is because when they are done the "old" say of doing them across a bench, the tendency is to drop the hips below the shoulders and this places far to much stress on the shoulder joint. If the hips are on the bench in the manner I described above the hips are either at shoulder height or even higher would be fine, as well. The bend at the elbow should be minimal and just enough to take stress off at the elbow. If you are hitting the floor with the dumbell, you have too much bend at the elbow and need to keep the arm more straight.
This is a great movement, in my opinion. However, on the rare occasion I see someone doing them it is usually as a chest exercise which is silly. The pec minor is certainly involved, as are the triceps, but they are not prime movers. If you keep the elbows in close to your ears as if you are trying to rub the sides of your head with your arms as you do the movement, you will quickly feel the stress around the shoulder blade, under the armpit where it should be.
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