Old sea captain's trick
The old sea captain's trick is a term coined at SL, for this situation where Black mistakenly responds to with the counter hane of , expecting White to block at , then connect at .
Instead, White cuts back at , allowing Black to capture at , then ataris on top with .
If Black now connects at , a white move at a will seal the fate of all the black stones involved in the skirmish.
The only thing Black can do is cut back again at b, and a picnic ko is the result, which is favorable for White.
Note that the marked White stone makes a difference: