The action of the switches on the bare skin would leave raw weals on the bottom cheeks. After about six strokes cutting of the skin and blood spots would occur where the switches had slashed too often. A well applied birching could leave permanent marking on the buttocks.
In the 1970s I remember hearing a radio interview with a lady who said been birched in an Scots approved school in the 1940s. She said she got it twice! Fist when whe was 12 and again when she was 14!. I remember she spoke with thick Glasgow accent. She said that her second time across the birching stool she got ten strokes it was the worst experience of life!
There must be a number of now elderly women around whose backsides bear telltale faint white scarring of a birching! A permanent reminder of their time at a Scottish approved school.