Sans sees the first swipe of an arm coming for the skyway, and knows that not everyone is going to be as quick to get out of the way as he could be. So, instead of teleporting away, he teleports into the direct path of that first arm.

He sticks out the hand with the scarab bracelet attached, and hopes for the best. It's meant to stop all momentum that touches the hand that bears it - he's tested it in less ridiculous circumstances. Will it work now? He hopes so, or he might end up being toast.

If it can throw off her balance at all...