The medium grip pullup is performed just like a regular pullup, but is done with a grip that is about shoulder width. Make sure that when you are performing the exercise, you think about start the movement by retracting your scapula, then pulling the elbows as far back as possible. You also want to arch your upper back hard and try to touch your chest between your collar bones and the nipple line.
This exercise is awesome in general, and one of the best back exercises out there that doesn't include the word "deadlift" somewhere in it. This variation also has great carryover to your squat, as it really helps strengthen the muscles that are responsible for having and holding a tight arch in your upper back. They will also help improve your lockout strength in the deadlift.
