Sans had just been getting ready to go for Peridot if she went on the attack, so he's ready and waiting when Jasper loses patience. The trouble, though, is in how boldly she went for it. There was so little hesitation, he barely has a moment to make a call.

He lunges forward, jaw open, to try to clamp down on Jasper and pull her back - restrain her, poof her, he doesn't care. Whatever will keep her from unleashing hell on everyone else.

He's just not fast enough.