I have only heard two cases of this, both clothed as far as I know.
In the first case (at my prep school). A senior boy (12-13) told me he had been given a repeat six (for turning off the school electricity) when he was found to have padded his thick flannel trousers. As far as I am aware, the cane used was probably no more than 60cm in length and light weight. Despite the usual necessity to remove underpants for games and PE, I never saw any cane marks on any boy at that school as canings were not common (maybe half a dozen a term) and not severe as George describes.