Expand the ribcage, stretch the lats.
Keep your hips down and elbows fixed, the movement comes from the shoulders.
Lowering the dumbbell behind your head with fixed elbows takes the lats through one of the longest loaded stretches available in the gym, and pulling it back over your chest trains them through a big shoulder arc. The chest assists through the same path, which is why the pullover has always straddled back and chest day. It is the rare move that loads the lats in a lengthened position without a bar or cable in sight.
It works both along the same arc, but the lats lead here. Keeping a slight elbow bend fixed and thinking about pulling with your armpits biases the back, while flaring the elbows and bending them more turns it chest-heavy. Decide which you are training and cue accordingly.
Both work. Across the bench with hips dropped gives a slightly deeper stretch and a bigger range, along the bench is more stable and easier to learn. Whichever you pick, keep your hips down so your spine does not turn the stretch into an arch.
Moderate. The stretched position behind your head is the productive part and also the part that punishes overloading, so use a weight you can lower slowly to a deep stretch without the elbows bending to bail you out.
