Which muscles are worked?

This move primarily targets the upper body, including:

Pectoralis major (chest)
Latissimus doors (back)
Triceps brachii (back of the upper arm)

How to do a dumbbell pullover

This is all about targetting the backs of the arms, as well as getting the chest working. You should feel the whole of the upper body working to stabilise you.

Lie on the floor with your knees bent and feet flat on the ground.
Take your dumbbell and hold it so that the palms of your hands are supporting one end, fingers and thumbs knitted or overlapping.
Extend the arms over your head and then move them behind your head to lower the weight towards the floor.
Once it reaches or nearly reaches the ground, slowly bring it back up and over so that your arms return to being straight above your face.

Dive into dumbbell pullovers for toned triceps