Open up the chest and ribcage with a deep stretch.
Keep your hips down and ribs from flaring. Move only at the shoulders.
The pullover is the rare move that loads the chest and the back through the same arc, the pecs and lats both attach to the upper arm and both get stretched hard as the dumbbell travels behind your head. That deep overhead stretch with the ribcage expanded is a position almost nothing else in the gym trains. It earns its place as an upper-body finisher that opens the chest, stretches the lats and hammers the often-skipped overhead range.
Both, and you can nudge the emphasis. Keeping the elbows closer and thinking about pulling with the armpits biases the lats, while a slightly wider elbow position and squeezing the chest as the weight comes over biases the pecs. Either way both get a serious stretch.
To the point of a strong stretch through the chest and lats, with your ribs still down and hips on the bench. If your lower back arches off the bench to get more range, you have gone past your useful range.
Moderate. The stretched position puts the shoulders at a long leverage, so start lighter than feels impressive, own the arc with a slight fixed elbow bend, and add weight only when every rep is smooth.
