A ladder-breaker is a stone in the path of a ladder that decides whether or not the ladder will work for capturing. It can refer to either an existing stone on the board, or to a move made after the ladder has already been set up.
In the case of stones already on the board, the presence or absence of the ladder breaker can be an important factor in choosing a line of play. Many joseki books, for example, will note that certain joseki variations are only playable if the resulting ladder is favorable.
When a ladder-breaker is played in the presence of a pre-existing ladder, it is often a dual purpose move, similar to a ko threat, which either allows the player to resolve the ladder in his favor or makes a local gain nearby. The opponent must then decide whether answer the ladder-breaker or resolve the ladder, which can be a serious dilemma. For this reason it is often said that a player should capture stones caught in a ladder at the earliest opportunity. The possibility of ladder breakers is also the reason that it is generally preferred to capture stones in a net rather than in a ladder. See also ladder versus net.
A player may play or simply have several types of ladder breakers. The position of the ladder breaker tells the method of escape; it also has an influence over when that particular ladder breaker works. For example, cutting ladder breakers do not work if the stone to be cut has a diagonal connection away from the ladder. Other types of ladder breakers may not work at all near the side or corners.
We will discuss three types of ladder breakers. The types played along the Circle marked points in White's path; the type played along the Square marked points in Black's path; and the type played at the points marked a to cut.
An inline ladder breaker functions as two consecutive plays out of the ladder. It produces the extension, an easy escape. The inline ladder lies on the circle marked points.
Black plays at b; however, at a breaks the ladder. at b invalidates this defense, becoming a piece of the ladder.
Notice that this stone connects to by Diagonal, becoming impossible to cut. Black should produce a net if possible.
Also notice that White gains three liberties by this ladder breaker.
The defensive ladder breaker blocks Black's path. By blocking the offense, it provides a defense rather than allowing White to run.
JohnMoser: What does this say about territory influence if Black plays the ladder out?
Notice immediately that White gains four liberties from this ladder breaker.
The offensive ladder breaker attacks Black's attacking stones. Playing to the attack ends in close combat.
Notice White places into atari with . would further place into atari. White at a would capture , while b would capture .
In this particular sequence, White lives immediately; in many situations, White has a difficult fight. In some cases Black can still kill the White group, for example if the ladder is enclosed in Black territory.
Note that Black in the Offensive ladder breaker path provides only partial protection: further playing out the ladder will capture unless Black also has a stone at a. By contrast, notice the defensive ladder breaker (lower right) produces a capture if a Black stone precedes it. The inline path provides more complex considerations.
Interestingly this means that White may play a ladder breaker by threatening Black elsewhere and being driven away from the ladder. A Defensive ladder breaker will not help, as Black will capture; but the Offensive ladder breaker will, if the fight doesn't put a Black stone on the Defensive path.
JohnMoser: I could write a lot of examples about playing ladder breakers; but the reality is that eventually a fight becomes bulky enough that you need to extend toward your laddered stones with your last threat, and get a non-interfering answer. The long and short of it is to try to get influence and possible life if being driven through your ladder's path, then tenuki to the ladder. You might lose your attack; but if you can live over there and run your escaped ladder towards life, you'll significantly reduce your opponent's territory. This is why you net or capture ladders as soon as possible.
For example here is much too violent to be useful as an approach move: the stone will suffer from a local disadvantage if Black answers here.
On the other hand if Black has to ignore for tactical reasons, and White can play hane next at a or b, White will have the upper hand locally here.
There is an overlap between plays that make good ladder-breakers and those that are good ko threats, therefore. And if these plays are ignored, the same sorts of local follow-ups will apply. In general fighting a ko is much more complex in its implications.
There is a difference, though, when one considers what the other side (Black in this case) can do to resist. In the case of ko fights, answering is just what the opponent counts on.
Here it would depend on the actual ladder whether made the ladder good again for Black. If White continued at c in another attempt to break the ladder, the game will undoubtedly become difficult. These resistance fights are hardly mentioned in books: probably they are an aspect of pure strength.