As you increase your weight in your deadlift or in Olympic lifts like cleans and snatches, grip becomes a limiting factor. As the weight gets heavier, the bar has a tendency to roll in your hands, making holding onto it for the duration of the lift difficult or even impossible. One way to counter the limiting grip factor on the deadlift is to assume an alternate grip — or “mixed grip” — with one hand supinated (underhand) and one hand pronated (overhand). As the bar starts to roll out of the fingers of the pronated hand, it is at the same time rolling into the fingers of the supinated hand, and vice versa.
The disadvantage of the mixed grip is that it loads your shoulders asymmetrically. Your prone arm is held in internal rotation and the supine side is in external rotation. Often this causes a “windmilling” effect on the supine hand in which the barbell rotates away from you during the pull. Windmilling makes the lift less efficient and much more difficult as you now have to spend extra effort controlling the bar. A swinging bar can also pull your back out of extension as the bar will suddenly become more difficult to lift.