I was riding down a hill once, and my lockring stripped and came off (so the cog followed). I just put my foot on the back tire and stopped. It was a little un-nerving, but that's how we used to stop when I rode brakeless BMX. As far as skidding without clips, skipping seems to be impossible on platforms. I don't even ride my track bike without clipless anymore, clips don't seem to offer nearly as much control.