Put her in a position that’s impossible to hold. Make her kneel. Have her spread her legs, and rise on her haunches just an inch or two, so that her arse isn’t resting on her ankles. Have her knit her fingers behind her head, and keep her elbows out, in an even plane with her shoulders. Put your hands on her. Adjust her posture until it is exactly as you want it.
Then retreat. Sit down somewhere in her eyeline, and wait. Do not tell her how long she must remain this way. Tell her that she must stay until you say otherwise. Watch her face.
Let a minute pass. Let another. She will waver. Her arms will move in small increments, tired shoulders pulling them back into place. Her legs will shake with effort. She will tense the muscles in her stomach. Breathe very stiffly.
She will correct herself whenever she flags, if she can. She will exhale through her nose – sharp, staccato breaths. Say nothing unless she lets herself drift. Correct her if this happens. It should not need more than a couple of words.