It is a common mistake when doing a penché in ballet to lead with the upper body. Since a penché is typically done in arabesque, a dancer can sometimes get caught up in trying to get their back leg to a 180 degree position and lead with their upper body in an effort to get their legs more split. A proper and clean arabesque penché is actually done by leading with the arabesque leg and leaving or holding your back against your leg. It’s essentially the same position as an arabesque, just tilted over your standing leg. The position of the back should never change.
If you follow this tip and don’t find your legs getting to your usual straight 180 degree line, this means there is an issue of strength in your back and core muscles. Continue trying to keep that back and arabesque leg in the same position and you’ll be on your way to a better more classically looking penché!