The arm strikes Sans's hand, and amazingly, it does stop it in its path for a moment. Yet, as anything with enough momentum hitting an obstacle, it doesn't stop it for good. Instead, it slips upwards and off of Sans's hand, trying to regain momentum, and ultimate swooping him down onto the surface of the skyway.

Still, at least that arm gets redirected upwards and away from the pack. But there are more coming.