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[go]$$ White is at loss for a reply.
$$ -----------------------
$$ . . . . . . . . . . . |
$$ . . . . . . . . . . . |
$$ . . . . a 2 1 3 . . . |
$$ . X . . 4 X O X . . . |
$$ . . . . . . O 5 . . . |
$$ . . . . . . . b O . . |
$$ . . . . . . . . . . . |[/go]

Cutting on the outside with White 2 is reckless. White is at loss for a reply when Black pushes out at 5, and if he plays 4 at 'a', then his position collapses after Black 5, White 'b', Black 4. This is good for Black without question.
(Lessons in the Fundamentals of Go; Toshiro Kageyama 7-Dan)

This may be good for Black without question for Toshiro Kageyama, whereas for me there is a small question.

Why is White at loss for a reply?
What is going to happen if White simply captures Black's stone?

Click Here To Show Diagram Code

[go]$$ Capturing Black's stone.
$$ -----------------------
$$ . . . . . . . . . . . |
$$ . . . . . . . . . . . |
$$ . . . . a 2 1 3 . . . |
$$ . X . . 4 M O X . . . |
$$ . . . . . 6 O 5 . . . |
$$ . . . . . . . b O . . |
$$ . . . . . . . . . . . |[/go]

Thanks.

Click Here To Show Diagram Code

[go]$$ Capturing Black's stone.
$$ -----------------------
$$ . . . . . . . . . . . |
$$ . . . . . . . . . . . |
$$ . . . . a 2 1 3 . . . |
$$ . X . . 4 M O X . . . |
$$ . . . . . 6 W 5 . . . |
$$ . . . . . . . 7 W . . |
$$ . . . . . . . . . . . |[/go]

Black pokes out and white's stone on the right is severely blighted, the classic broken knight's move shape. Black now has a big corner and is strong poking out into the centre.

Click Here To Show Diagram Code

[go]$$
$$ -----------------------
$$ . . . . . . . . . |
$$ . . . . . . . . . |
$$ . . . . . . . . . |
$$ . . . . . . . . . |
$$ . . . . W X . . . |
$$ . . . . . B W . . |
$$ . . . . . . . . . |[/go]

Generally, any time you see the shape of the marked white stones, you should think about the black push--it should be your first instinct. In isolation, it's incredibly powerful.

There are exceptions, and you should read as much as you can, to figure out if you're witnessing one of those exceptions. In fact, as you get stronger, you'll see the exceptions more and more often, and the normal case less. Why? Well, if your opponent is any good, he won't let you have such a powerful move willingly. It's the same reason that strong players more often sacrifice stones than get them killed, and the same reason you rarely see $100 bills laying on the street.

But at your level, you'll see the potential for these moves all the time, and it will help tremendously.

There are a few more (even more) basic examples at http://senseis.xmp.net/?BasicInstinct.

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Occupy Babel!