it happens pretty often in "with anyone" and some free-for-alls... but i never use sing in 1v1, so... i'm not sure about it's 1v1 uses.
anyway, basically if you catch someone in sing while they are moving fast, they will slide away asleep. like i think if they're hit by a jiggs/fox dsmash (the kind that make you slide across the ground), then you get hit by rest, you will slide while asleep... but you see, in this case it must be more than two players (has to be one random opponent, one jiggs, and one slide-causing opponent).
also, i think this worked one time with the checkpoint marker in greenhill zone and one time with wobboffet... i'm not sure how, though, because it was really fast.
i've also seen it work a fair amount of times after an opponent hits a sideways spring while you're singing.
either way, i don't see it very useful unless the opponent can fall of the stage while asleep or if it's a walk-off stage (causing them to slide to doom)... because they slide so far away from jiggs that i don't think it's possible to follow up with an attack.