Free Atari

Free atari  

When mistakenly descends here, is a free atari. The marked stone is making this atari "free" because White doesn't lose the option of atari from the other side.

Not free  

Suppose the marked stone is not there. Now the atari at A would not be free because White loses the atari at B. In this case, it's better not to play the atari.

Not forcing  

If here and Black ignores, later may not be forcing as in the first diagram. Black could decide to dodge at or ...

Ko  

... play a ko.

Black patches up in gote  

Black may also decide to still patch up his corner, which was the right thing to do at in the first place. See Beware of going back to patch up.