Barry giving Killer Frost the patented Barry Allen eyes of “I believe in you, I believe in me, I believe in us” melted her villainous impulses long enough for Caitlin to return and help save Wally (Keiynan Lonsdale) from vibrating out of control.
The real stopping of Killer Frost came from Barry tapping into his friendship with Caitlin.
In The Flash episode “Killer Frost,” H.R. (Tom Cavanagh) posited that Barry’s (Grant Gustin) true superpower is hope and that the guy just has great reservoirs of it within himself.
The Flash has never made it a secret that this particular concept is what drives the show.