One of the most common mistakes with kettlebell swings is doing a "squatty" swing where the kettlebell goes below the knees. That's the incorrect form and you won't get as much out of the exercise. It's also rough on the lower back.
Instead, you're looking for a hip snap or hip hinge movement where the 'bell stays close to the body... close to the crotch to be specific. "Attack the zipper" as Coach Dan John likes to say.
To fix this issue (or make a new kettlebell swinger aware of it to begin with), I like to set up a foam block or similar underneath the athlete. Go too low and the athlete will hit the block. Immediate form feedback!
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