That’s a prison strap of the sort that was used in Canada and many U.S. states (I don’t know about the federal system) up until the late twentieth century. You’ll see a similar strap used in the excellent movie “Brubaker” with Robert Redford. AFAIK it was typically applied to the buttocks, where it could produce the desired pain without the risk of internal damage. One could argue whether the strapping depicted here is cruel (if it were real) but it is dangerous.