They require a specialty bar - a safety squat bar — which you don't have to hold onto to keep on your back. (onnit.com)
Another critical mistake I see is people dropping their hips too low and basically trying to squat the bar off the ground. (bodybuilding.com)
The high bar squat bar placement on the shoulders is closely connected with the high bar squat elbow position and forearm position. (christianbosse.com)