I've had soapsticks inserted during a spanking and before a big soapy enema. They sting and really get my attention. But the worst is when I must carve a butt plug out of a new bar of Ivory - maintaining the entire width of the bar - wet it, insert it and stand in the corner to wait for my spanking. It begins to sting at once and soon builds until it's stinging terribly and all I can think about is, "How did I get myself into such a predicament? Owwie oww owwie. Let's get this spanking over with so I can get this f*cking soap out of me!!"
The soap butt plug's much worse than the fat ginger figs I've endured over the years (being the one doing the carving has always had a strong effect) and thankfully it's reserved for "special occasions" - when restraints are used and the spanking goes well past "funnsies".