One arm, constant cable tension on the rear delt.
Move only at the shoulder, keep your torso still so the rear delt does the work.
Standing side-on and sweeping one arm out across your body puts the rear delt under cable tension through its entire arc, including the start position where dumbbells give a rear delt nothing. The mid-back assists as the arm travels wide, but a still torso keeps the credit with the shoulder. It earns its place because rear delts are chronically undertrained, and constant tension with one strict arm is one of the best ways to fix that.
The cable loads the rear delt even at the start of the rep, where a dumbbell hangs slack, and standing upright is easier to hold than a long hinged position. You trade nothing except the excuse to swing.
The rear delt is a small muscle at the end of a long lever. Go heavy and your traps, back and momentum take over instantly. Light weight, slow arc, hard squeeze.
In the back of the working shoulder, behind the joint. If your traps or lower back are lighting up, slow down, reduce the stack, and keep your torso completely still.
