Someone mentioned pinning their arms over their heard and I did like that too. The idea of sitting on their arms and going to town on them. Unable to do anything but scream and cackle as your tools, be they fingers, feathers, brushes etc, poke, rub , stroke and glide against their defenceless underarms. Their giggles and pleading for mercy broken every so often by grunting as they try desperately to pull their arms down to defend their helpless pits from those godforsaken sensations being forced from them to their brain. Their hips bucking up and down, rising up and slamming right back into the floor with am audible thud while their unbound legs are free to kick and flail in every direction, probably knowing deep down that it won't do anything to ease the attack on their sensitive underarms but still being compelled to do so anyway.