Young, fairly inexperienced, climber descending on autobelay. Extended legs during decent and one leg went between the hanging ropes from the neighbouring route. These ropes were loosely tied as a slipknot to keep them off the ground. Once the descender's leg reached that knot, their knee got stuck and they pivoted upside down from their knee to reached the floor head first. (Still attached to the autobelay so no hard impact.)