Historically speaking, Sans's primarily skill in combat has been that he is extremely difficult to catch. One hit is more than capable of putting him down - the main challenge is ever getting it in. But that's when he's thinking, when he's physically and mentally at the ready. Right now, Sans is neither of these things. He's just reacting, out of anger and out of fear.

So, while he's busy reacting to Rarity crying out to him in fearful tones, Lapis's attack effortlessly lands. He goes from being distressed that his blaster is being nullified to realizing that he's already completely lost. He's sure of it, the moment the water chains lock him down.

He snaps back into the wall, pinned by his arms and his neck. The blaster at his side struggles against Rarity's telekinetic smothering, but can't get the mobility it needs to fire back, despite the cyan glow spilling between its teeth and locked jaw. Sans's reaction to being bound is utterly without pride. He squirms against Lapis's hold for only a few moments before his sockets go wide and filled with dread - his terror is wild and unthinking. Context means nothing to the situation, all he can think about is how he's trapped again, how he messed up again, and how he's going to be hurt.

He yells aimlessly, the sound pilled with pain, and another blaster gets summoned through a mixture of will and instinct. It immediately lines up a shot on Lapis.