If she flinches and squirms, she’ll wiggle her butt a little.
Perhaps that was the tease and provocation that led to this in the first place. That, and being over-confident that in that outfit, pulling her skirt up would be too awkward in public.
"...the girls are strapped to the bare military metal beds in an extreme position, with their feet by their heads, spread wide, so their butts, pussies and anuses are fully accessible..."
If she flinches and squirms, she’ll wiggle her butt a little.
Perhaps that was the tease and provocation that led to this in the first place. That, and being over-confident that in that outfit, pulling her skirt up would be too awkward in public.