Frisk reacts on pure instinct, trying to stop but also turn in order to help the other-kid-that's-falling.

It doesn't work. They skid as they twist, flinging an arm out in half an attempt to catch them even though they're falling now, too.

It leads to a movement so smooth it almost looks purposeful, legs shooting off to one side and an arm stretching out the other until they land on their chest hard enough to drive the wind out of them completely.