Can someone explain how to tell where I should throw in/when to throw in? I hear players talk about it all the time, and I always have a hard time understanding when I should go for it in my games.

Throw in to
- eliminate an eye if thereby the group is then not alive,
- connect the throw-in string if it wins the capturing race,
- accelerate reduction of approach liberties in a capturing race,
- create a liberty shortage,
- connect strings making them strong (thick),
- prepare a squeeze,
- prepare a semedori,
- threaten to do some of the above if the opponent's reply cannot defend against all threats because of multiple threats or ko,
- etc.

EDIT

Well, what are you trying to do? A throw-in is often used to make a false eye, so if there's a group you are killing and you read the throw-in kills it then play it. Or they can be used to reduce liberties (e.g. on first line), so if you read the throw-in reduces their liberties and lets you capture some stones you want to capture then do it. A snapback is another example of a throw-in tactic taking away liberties and often come up as a tesuji to capture some cutting stones. I wouldn't approach a game as "today I will make 5 throw-ins", but if you post a game of yours for review I can probably find some opportunities for throw-ins as they are a fairly common technique.

Most good tsumego series for beginners will drill you on the throw-in technique by covering many, many typical situations. If you want to develop your intuition for it I would recommend the Level Up & Jump Level Up! series.

The sequence to kill the J group is a good example of using a throw-in to eliminate eye space. In the second diagram in the following webpage, move 7 is the throw-in.

Site: http://senseis.xmp.net/?JGroup

Here are a few other examples of throw-ins: http://senseis.xmp.net/?ThrowIn

Basically, it's sacrificing a stone inside your opponent's group to try and either kill it (usually), or make profit out of it that's greater than the sacrifice value.

Throwing in happens in joseki, too. Here's a familiar one:

Click Here To Show Diagram Code

[go]$$B joseki beginning
$$ ------------------
$$ | . . . . . . . . .
$$ | . . . . . . . . .
$$ | . . 4 . . . . . .
$$ | . . 1 , 2 5 . . .
$$ | . . . . 3 . . . .
$$ | . . . . . . . . .
$$ | . . . . . . . . .
$$ | . . . . . . . . .
$$ | . . . . . . . . .[/go]

Click Here To Show Diagram Code

[go]$$W joseki continuation
$$ ------------------
$$ | . . . . . . . . .
$$ | . . . . 6 . . . .
$$ | . 5 O 8 . . . . .
$$ | 9 4 X 1 O X . . .
$$ | . 7 3 2 X . . . .
$$ | . . . . . . . . .
$$ | . . . . . . . . .
$$ | . . . . . . . . .
$$ | . . . . . . . . .[/go]

WHite 10 throws in to the left of W1 in the second diagram to set up a squeeze.

Here is an example of the throw-in to take away a potential eye and kill a group.

Click Here To Show Diagram Code

[go]$$B Black to play and kill
$$ -------------------------------
$$ . . . . O . X X . O B 1 2 . . .
$$ . . . X O O . O O O O O X . . .
$$ . . X . X O O X O X . X . . . .
$$ . . . . X X X X X X . . X . . .
$$ . . . . . . . . . . . . . . . .
$$ . . . . . . . . . . . . . . . .[/go]

White threatens to capture the stone and make a second eye. After at1 takes away that eye.

Here is a throw-in to reduce liberties and capture stones.

Click Here To Show Diagram Code

[go]$$B
$$ | . . . . . . . .
$$ | . . X X . . . .
$$ | . X O 4 5 . . .
$$ | . X O 1 O . . .
$$ | . X O 2 3 . . .
$$ | . . X X . . . .
$$ | . . . . . . . .[/go]

One to remember, called the Cranes’ Nest.

Edit: Also, here is a throw-in in a strange corner sequence.



White has horrible shape.

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