Sans can't help but cringe at the sound of that crack. It's a sound that's a little too familiar to him, still.

Then he looks up at Frisk. Then he realizes that that sound wasn't coming from him. Then Frisk is falling and oh god...

About halfway down the branches, Frisk's fall will halt quite abruptly. This is because their soul has just turned blue. A look up Sans will reveal that his eyelights are glowing a little brighter, smoky edges of magic trailing out of the sockets.

"...yeah," he finishes, somewhat awkwardly. It's barely any effort at all to right Frisk in midair and set them down carefully on their feet. "Careful."