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The front squat requires you to keep your upper body more upright. A forward lean will result in not being able to hold the bar on the front of the shoulders. – Christian Bosse

The overhead squat requires a more upright posture than the back squat and you're forced to execute the squatting pattern precisely, otherwise the bar would drop to the front or back. You can see the high concentration levels of the athlete and accurate execution. Mastering the overhead squat can help your back squat technique. ­– Christian Bosse