what in the world is a butt wink ?

The butt wink 

isn't the name of a weird sex position or new dance move. It's actually a term for when your tailbone tucks underneath your body during a squat. "When your squat reaches a certain depth, the range of motion in your hips may become limited," says Sean De Wispelaere, a Thrive Training Systems coach. "To compensate, your pelvis tilts backward and pulls your lower back with it." Hence, the wink. Watch the video below for a quick demonstration.

Without weight, the butt wink isn't a cause for alarm, says De Wispelaere. However, adding weight to the movement puts your spine in the danger zone. Here's why: The weight compresses the disks in your lower back. Then, rounding your lower back—like you do when you butt wink—increases the pressure to these disks, moving them out of alignment and making them vulnerable to herniation. 

It helps to have someone check your form, so ask a friend or trainer at the gym to watch for your butt wink. If you're alone, perform a squat in front of a mirror. Take note of the moment your tailbone starts to tuck underneath you. "Stop right before that moment every time," says De Wispelaere.

Your anatomy has a lot to do with when your butt wink actually happens during a squat. Even so, there are ways to improve it so you can eventually reach your maximum squat depth sans winking. Want to learn how? Sports biomechanist Bret Contreras, C.S.C.S., created a video about the science behind the butt wink, complete with demonstrations from "Skelly," a skeleton model from Costco.