The elbow literally cracks Sans in the face just as he's pulling them through the shortcut - meaning they are pulled out considerably earlier than he was aiming for. He tumbles back as soon as they've entered back into normal space time, in what appears to be a poorly populated courtyard area of one of the lush buildings nearby.

There's a thin crackline up the bridge of Sans's nose, and there's a moment where all he can do is clutch his face in apparent shock, squirming back from Greg defensively.