We must assume, from her posture, from the proud look on her face, from the title of the book where this illustration appears, and most especially from the fact that the illustration is by Louis Malteste, that the young woman on the chair is attempting to show off some spanking welts under all those bustled-up petticoats:

showing off her spanking welts

The artwork is from Quinze Ans – Roman sur la discipline familiale suivi de quelques lettres sur les châtiments corporels dans l’éducation des jeunes filles et de Sonia ou la Belle Etudiante (1927). (Google Translate renders that title “Fifteen Years – Novel on family discipline followed by some letters on corporal punishment in the education of young girls and Sonia or the Beautiful Student”.)

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