Frisk wanted to get back fast, and admittedly they did aim a little when they saw Lapis holding Sans down, but at the same time they didn't really anticipate the sheer force of it. They tuck instinctively into Lapis's body as she curls around them, shielding their head and neck from damage. They know how to fall. She didn't have to do that.

The impact is still jarring. It's a moment before they untangle themself from Lapis, mud squelching between their fingers as they push themself upright. They shift aside off of Lapis, but shrink down a little at the sight of the blaster aimed at them. At her.

"Don't - " Lapis still remembers somewhere. She just tried to protect them. Frisk's got enough sense to scoot back from her a little before trying to stand, but they lift a hand as if to ward off Sans. Let's just see what happens.