While she's restrained spread-eagled, I put a belt around Sierra's waist and use it to run a rubber band between her legs. It's much easier to hit her pubic mount with it that way than using the ones running all the way around her body. She figures out quickly that she can rotate to protect her insides though, even with a spreader bar between her knees. Eventually, I resort to coercion: snapping a rubber band high on her side can hurt even more. With that as a threat, I make her hold her legs open for me to pull the rubber band down and snap it directly up into her cunt.