Check-Swing Strike

If the home plate umpire grants a check swing appeal, and the base umpire thinks that the batter did, in fact, swing, they will overturn the original call, and call a strike. They will do so by signaling with a clenched fist, and a motion that has combination fist-pump-and-stabbing-motion energy (similar to calling a runner out in other contexts). The base umpire’s judgement is final, so now a swinging strike is added to the count.