When you block a poke and you poke back.
For example, if you block Sonya's d1, she's at -12, meaning you can poke back with a d1 or a d3 that's faster than 18 frames (her fastest move is 6 frames, so even if she does that you have an 18 frame window to do whatever you want, unless she jumps out but whatever). It's usually pokes because they're the fastest moves in the game.