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 move on the row below the outpost, in the direction of where the ladder will be coming from. Are there other foils?
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 do foils work?
There is a simple set of rules telling when a ladder escape can be foiled.
- Ladder escapes on the second row cannot be foiled.
- Ladder escapes on the third row can be foiled if the cell adjacent to it in the direction from which the ladder will be coming is occupied by the other player:
Ladder escapes on the fourth row can be foiled if one of the two cells in front of it is occupied by the other player:
If the ladder coming from the left is a 2-ladder, Blue can foil the ladder escape only if he already occupies the cell marked (*).
If the ladder is a 3-ladder, then Blue can foil the ladder escape if he already occupies at least one of (*) and (+).