Consider the following position:
Red has just played f6. In his next move he can either start a ladder at c7, using f6 as a ladder escape, or he can play g4, making an unbreakable connection from top to bottom. Thus f6 threatens two different connections.
However it does not secure Red a connection, because there is one vulnerable cell, namely e7:
So to foil a ladder escape you make a move on the row below the outpost, in the direction of where the ladder will be coming from. Are there other ways to foil?
Foiling does not always work
Consider the following position, which is almost equal the one in the first diagram:
If Blue tries to foil f6 now, Red responds at f7:
Observe that the ladder still works, and so does the connection via g4. Since Blue only can stop one of these two, Red wins.
When does foiling work?
In general, it is difficult to figure out when a ladder escape can be foiled. There are some simple rules that apply in some cases.
- A ladder escape fork on the second row is unfoilable.
- A ladder escape fork on the third row is unfoilable if the cell marked "*" is empty, and is not required for the "connection up":
However, if the cell marked "*" is occupied by Blue, the ladder escape fork can often be foiled; in that case, playing at "+" is the only way of foiling it. Also, if the cell marked "*" is empty, but is required for Red's threatened upward connection, the fork may be foilable by playing at "+", as in the following example:
Because the cell marked "*" is required for Red's threatened connection to 10, the ladder escape fork is foilable by playing at "+" (but not by playing at "*").
- A ladder escape fork on the fourth row is more complicated. If a 2nd row ladder is already approaching, the fork is unfoilable if the cells marked "*" both are empty (and not required for the "connection up"). Otherwise, it may be foilable, and in that case, playing in one of the cells marked "+" is the only way to foil it.
If the approaching ladder is a 3rd row ladder, the fork is typically foilable by playing at "+".
The foil may not work if Red has a lot of space. For example, the following position is winning for Red (with Blue to move, and assuming "*" connects to the top edge), but Red needs at least the amount of space shown. If any one of the empty cells is occupied by Blue, the position is foilable.
Playing a foilable move in the hope that the opponent doesn't know how to foil is sometimes called a fishing move. The terminology originated with French-speaking Hex players, such as Mickaël Launay in this video, where it is called le coup du pêcheur, literally "the fisherman's move". A fishing move is a kind of trap that is sometimes effective against beginners, but should not be used against experienced players, as it is usually bad for the player who makes it. A fishing move can also sometimes be a last ditch effort by a player who is losing and is desperate for the opponent to make a mistake.
Fishing moves often take the form of playing a peep in an ascending bridge. In the following example, a red 2nd row ladder is approaching from the left, and the blue bridge is ascending (relative to the direction of the ladder).
Red plays the fishing move 1, hoping that Blue will defend the bridge and Red will get a 2nd row ladder escape. Instead, Blue should foil at 2.
Note that if Red follows through on the bridge threat, the result is a 3rd row ladder for Red, which is typically worse than the 2nd row ladder Red would have gotten otherwise. Playing a fishing move in an ascending bridge usually results in raising the ladder by one row, and is bad for the player who plays it.
On the other hand, playing a peep in a descending bridge is often useful and not a fishing move. It typically serves to lower the ladder by one row (for example converting a 4th row ladder to a 3rd row ladder), or to escape a 2nd row ladder outright. Consider the following example, with a red 3rd row ladder approaching from the left. Note that the blue bridges are descending (relative to the direction of the ladder).
Red's intrusion 3 lowers the ladder from 3rd row to 2nd row, and Red's 9 escapes it. (Moves 4, 5, 10, and 11 are not usually played, but have been included for clarity).
- Mickaël Launay, Le Jeu de Hex, Tactique et Strategie - Niveau 1, 6. Le coup du pêcheur, 2014.