Pinching in the hips during squats is one of the most common complaints I hear from CrossFitters and weightlifters. A variety of restrictions can lead to this form of hip impingement ranging from ankle mobility to core positioning and boney anatomy of the hip joint. In athletes without any glaring deficiencies to address first, the following stretch easily corrects the hip impingement in many of my athletes.
We will describe this stretch as if working on the left hip. First, have the athlete get on hand and knees. Then, extend the right leg and focus on having as much weight on the left knee as possible. The head of the femur (thigh bone) will be gently mobilized deeper into the hip joint. Maintain this position for 30 seconds to 1 minute before proceeding to rotate the leg up (bringing the foot towards the opposite shoulder). Finally, sit back until a stretch is felt in the buttocks. Spend ~1 minute per side stretching in this position (searching for areas of restriction).
Recheck your squat and enjoy the feel of (hopefully) impingement-free squatting!