Article: John Keate, Head Master from 1809 to 1834, took over at a time when discipline was poor. Anthony Chenevix-Trench, Head Master from 1964 to 1970, abolished the birch and replaced it with caning, also applied to the bare posterior, which he administered privately in his office. Chenevix-Trench also abolished corporal punishment administered by senior boys. Previously, House Captains were permitted to cane miscreants over the seat of the trousers. This was a routine occurrence, carried out privately with the boy bending over with his head under the edge of a table. Less common but more severe were the canings administered by Pop (see Eton Society below) in the form of a "Pop-Tanning", in which a large number of hard strokes were inflicted by the President of Pop in the presence of all Pop members (or, in earlier times, each member of Pop took it in turns to inflict a stroke). The culprit was summoned to appear in a pair of old trousers, as the caning would cut the cloth to shreds. This was the most severe form of physical punishment at Eton.

Now answer this question: Why was a subject ordered to appear to Pop-Tanning in old trousers?
the caning would cut the cloth to shreds