This is a very hard exercise if done properly.
You will use a great amount of energy doing this and build a lot of thickness in the lats and upper back.
A bigger stronger upper back will also help bring your bench press up.
Grasp a dumbbell you can handle easily.
Place your opposite hand and knee on a bench now keep your back as flat as possible.
Holding the dumbbell let your arm swing slightly forward so you can fell a stretch in your lats.
Now pulling the dumbbell to your waist (it should move in a slight ark) squeeze your lats as hard as you can for two to three seconds.
Slowly return the dumbbell to the starting position for a good stretch.