The butt wink is a term used to describe the posterior rotation of an athlete’s pelvis during a squat, i.e., your tailbone tucks underneath your body during the squat.
A butt wink can occur because the range of motion in your hips is limited once you reach a certain depth. It might also happen because you have tight hamstrings.
There is no doubt that a butt wink is bad. Kelly Starrett says that when you butt wink, the whole system is then compromised do to instability. Your core turns off, causing you to get loose in the bottom, which leaves you poorly braced.
This becomes really problematic when you have a loaded barbell on your shoulders because your risk of injury goes up.
How do you fix a butt wink?
First, address your posterior chain and hamstring mobility.
Starrett suggests squeezing your belly and your butt to stabilize your spine at the top of your squat. Then, initiate the squat by pushing your knees out laterally and your hamstrings, not your butt, back. You should drive your knees out as far laterally as possible, and screw your feet into the ground, creating torque as you get deeper in your squat. Imagine that you have a connected sheet of paper towel under both feet and you want to tear it in the middle.
But until you master the awareness to do this while banging out a set of squats, stop your squat where you begin to butt wink.
For some athletes this occurs when their hips are at ninety degrees, for others it doesn’t happen until they get below parallel. If you are unsure of where your tailbone starts to tuck under your body, ask a coach to watch you and place a box or medicine ball to stop your squat right before this point.
Stay there with the box or medicine ball for a while and really feel what your body is and is not doing to this point. Are you tight in the bottom? Is your back rounding? Then remove the object, squat to full depth, and see if it feels different to you. Try to recreate the feeling your body had before you began to butt wink. If you are unable to do this, go back up to the box or medicine ball and don’t go any deeper until you have worked on your mobility and can squat without a butt wink.