Hi there! Thank so much for submitting your question. It's a great question, and ties in nicely with last month's column. Let's start by defining the term "butt wink."
Butt wink is a common term for losing proper spinal positioning when squatting to depth (in gym terms, "in the hole" or "ass to grass"). Instead of maintaining a neutral or slightly extended lumbar spine, the lifter experiences posterior pelvic tilt and lumbar flexion at the lowest point of their squat. This makes the butt "wink" or tuck under (the photo below left shows proper form, photo on the right shows butt wink).