although total isolation of a muscle is not possible, pull-over is a great exercise to develop mainly the lats muscle but it should not be performed with dumbbells for the sake of our shoulder health, and also in order to get a good contraction. cables and bands put you in a safer position while giving you a better contraction and stretch, meaning the range of motion is also a lot better. if you are using a band, make sure you grab the bands equally so both sides feel the same tension and if you are using a cable, make sure you lay down exactly between them so you perform the exercise in a symmetric form.