She’s figured out that eventually, when the whip lands lower on her body, on her rump, it will wrap around again, touching her in a place she thought unimaginable to punish.
"...the girls are prepared in the reformatory’s punishment room, naked, lying on their backs on special benches, bound with their legs spread above their heads, shamefully showing the two holes usually hidden..."
She’s figured out that eventually, when the whip lands lower on her body, on her rump, it will wrap around again, touching her in a place she thought unimaginable to punish.