Open wide and build a strong, balanced upper back.
Keep your arms long and lead with your rear delts, no swinging or jerking.
The dumbbell back fly has you hinge to near parallel and sweep the weights out in a wide arc, putting the rear delts directly against gravity where they can't hide behind bigger muscles. The long-arm sweep with soft elbows keeps the lever honest, so light dumbbells deliver a serious hit to the back of the shoulder. It's the free-weight answer to rear delts that pressing and lat work leave untouched, and it needs nothing but a pair of dumbbells and a flat back.
The long arm is a brutal lever and the rear delt is a small muscle. Heavy dumbbells just recruit momentum and turn the fly into a row, light weight with a pause at the top is the productive version.
Let them come together naturally at the top, but don't lead with them. Drive the arc from the back of your shoulders, if the rep starts with a blade pinch, it's become an upper-back move.
Yes, hinged over the end of a bench works well and takes your lower back out of holding the position. The arc and the soft elbows stay exactly the same.
