You can also tell when it starts to hurt… their voice breaks a little. What seemed like a good idea beforehand, rarely does so after a dozen or so strokes.
Both my wife & mother-in-law, strip me, put me over their knees for a hand spanking & paddling, then put me over the back of our living room couch as they paddle & strap me till my legs are kicking, begging, & promising them anything, if they'd just let me up. It gets everyone worked up & they have their way with me, to all our satisfaction.