is counter-atari, responding to the atari of by giving atari back. This is often a better alternative than the heavy connection at a.
naturally captures, after which (or a, depending on the overall position) gives atari. then connects.
Instead of connecting, here starts a huge ko fight, potentially a game-deciding ko. White will likely ignore any ko threat and resolve the ko with .
For White to instead connect with here is the farmer's hat, an empty triangle of a very heavy kind. This is rarely a good play.
Black will have a pincer stone at one of the points a to f. With a close pincer (a or b) White now usually connects solidly at p, with the aim of extending to the marked point. With the more distant pincers d and e White makes the trumpet connection at q for better shape (the other two pincers are rare in this pattern).