I certainly agree that the most effective slippers were the ones with very flexible soles. The gym shoes that are on the market these days are far too heavy and rigid. The ones that really hurt at school had a thin, smooth, flexible sole and a very light canvas upper. One trick used by some teachers at my school was to cut away the upper or part of it. In my article, I mentioned “Hotspur” which was easier for the teacher to grip because the canvas around the heel had been cut away.