Build the delts most people forget.
Keep your neck neutral and resist swinging, let the rear delts do the lifting.
Hinged over with the dumbbells hanging under your chest, this fly puts the rear delts in charge of dragging the weights out against gravity, exactly the plane they are built for. The mid back joins in as your shoulder blades draw together at the top, which makes it a posture move as much as a shoulder move. If your pressing volume dwarfs your pulling, this is cheap insurance for balanced shoulders.
Lighter than your ego wants. Rear delts are small and the lever is long, so heavy dumbbells just turn it into a swinging row. Pick a weight you can pause at the top with.
You are probably shrugging as you lift. Keep your neck long, lead with the elbows and stop when your arms are level with your torso, going higher hands the work to your traps.
Dumbbells demand more control and hit the rear delts hardest in the top half of the arc. The machine keeps tension through the whole range and is easier to load. Most people benefit from both across the week.
