This is a variation of the pull-up that removes the bar as an obstacle to your path. By gripping on two bars and pulling yourself up directly in between them, you hit a movement pattern that is halfway between a front pull-up and rear pull-up but without the shoulder stress associated with a rear pull-up.

For this exercise, you will need power rack and two Olympic bars.

The set up is VERY simple: set the safety rails as high as you can in the rack.
Now set two Olympic bars on those safety rails about 2 to 3 feet apart. You're all set!

To execute this exercise, place yourself directly in the center of the two bars.

You will be facing to the side of the rack. Grip both of the bars with a palms-facing-in grip.

Lift your feet off the ground (if you rack is not so tall, you can cross your feet behind you or just lift your knees up) and pull yourself straight up in between the bars.

Keep your elbows pointing directly to the sides to get the greatest effect from this exercise. This exercise operates in a similar movement pattern to the Behind-The-Neck Pull-Up (which is an exercise that is tough on the shoulders) but without any of the negatives associated with it.

When you're doing the exercise, make sure the bars aren't set too close together. This shifts too much of the focus of the exercise onto the biceps. If you are looking to work your biceps, you can certainly try this but in order to most effectively concentrate on your back muscles, keep the bars at least several feet apart.

Also, during the exercise, keep your elbows pointing directly to the sides. If you allow your body to move back, you will essentially be doing a front pull-up, which diminishes the effectiveness of the exercise.

If you don't have the strength to do a full pull-up with your whole bodyweight, try setting the bars a little lower so that you can leave your feet on the ground. Use your legs to help take up some of your bodyweight as you do the pull-up.

This can also be a good way to do a sort of drop set. Do a set of full weight pull-ups then keep going with your feet on the ground.

Overall, if you're looking for a way to shake up your pull-up training, THIS is a great exercise to do it with!