It's Chikusa's actions rather than his words that bring Ken up short, that have him stopping and looking back at Chikusa to make sure he's okay after that jump. (Ken knows as well as Chikusa that he doesn't have the same physical advantages Ken does. Even before channels, Ken's the more robust and athletic of the two by a long shot.) He's mad, but he doesn't want to see Chikusa hurt. If he did, he'd simply beat Chikusa up rather than seeking out targets elsewhere.

But, more than that...Chikusa has to be really upset or panicking to take such sudden, desperate action. And that gets Ken's attention.

"...what?" he asks eventually, once he's sure Chikusa hasn't snapped those stick legs of his.