Problem diagram.
Black plays.

Source:
Good and Bad Style
Go World #1, May-June 1977, p. 32

Solution follows    (or click here to download SGF)

Vulgar style.
Black 1 protects the cut and threatens to push out to the left along the top. What could possibly be wrong with this move? The answer is White 2, which stops Black and is now sente again. Black can defend with 3, but White would have yet another sente play at 4, to which Black would have to meekly defend at 5.

Black variation.
Black could defend at 3 instead, but White 4 seals off the top in sente by threatening A.

Correct.
Black 1 defends the cut, but now a White descent at A is no longer sente. White 2 can atari in sente, but Black simply connects at 3. Now White is faced with 2 bad alternatives: connect at A in gote, or even worse, let Black capture at A.

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