Sans doesn't really do anything when Lapis lets him go - at least, not besides slowly dripping down the wall (somewhat literally, thanks to the water left from his bonds) and sliding into a crumpled pile at its base. He wraps his arms around his knees and stays there like he's trying to block the rest of the world out.

What do you do when you've just endured something excruciating and you know there's only going to be more in the future? You do nothing. There's nothing you can do. Putting up any more of a fuss is pointless, and all it's going to do is cause you more hurt in the end.

So, he hides his face in his arms, his bones rattling with fear. He waits.