they'll break fall - they will try to spin around into a forward fall, then turtle up so you can't do anything to them. Eventually the ref will stand them back up because no progress can be made to to the faller turtling up. He can't just fall on his back - he'd lose the match.
That said, sure, something like hiza guruma won't hurt on a mat, but on concrete and if you don't know how to fall, it can be pretty painful I'd imagine. May not be enough to end the fight though.
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