Me and some buddies I used to ride with used to do this to each other once in a while, albeit this over a decade ago. We'd only do it to people we knew and trusted due to how close the bikes have to be to reach it. It's a pretty minor thing and sounds much worse than what really happens. Flip.. bike starts slowing some... flip back on.. back to normal. No clutching, keys or anything required.

I'm sure they didn't mean much by it, just goofing off, if you were comfortable with the closeness then it's a minor occurence. I wouldn't make too much of it, though it's stupid it's relatively harmless if skilled riders are involved. If however you were uncomfortable with it, asked them not to do it again and they did.... then go all slaphappy on them.