If I have a rear delt machine available I always go for it because EMG studies show it activates the posterior delts better than dumbbells or cables thanks to constant tension and less stabilizer involvement. I set the seat high so my hands stay slightly below shoulder height, I keep the scapula open and I stop the movement before the rhomboids and traps start helping. The rear delts move the arm, not the shoulder blades, so not retracting them is what keeps the tension exactly where it should be.

My second go-to when I don’t have the machine is a dumbbell rear delt fly superset with face pulls, which I picked up from a Hany Rambod and Jeremy Buendia training video and it also gave me great results.

at 12:55