A foldback, also known as folding under or a foldback underneath, is a situation in which a ladder changes direction and turns into a ladder closer to the edge. The attacker is still in control after the foldback. A foldback is not to be confused with a switchback, in which the ladder changes direction and continues further from the edge.
Foldbacks are only possible from 4th row ladders or higher, as there is not enough space under a 3rd row ladder to fold back.
Suppose Red has a 4th row ladder approaching a red stone at d5 (or the equivalent cell on the opposite side of the board):
In this situation, the stone on the 5th row does not act as a ladder escape. For example, Blue can block the ladder as follows:
However, Red can now get a foldback, namely a 2nd row ladder from right to left:
If Red has some way of escaping this ladder, Red connects.
Similarly, consider a single red stone at c4 (or the equivalent cell on the opposite side of the board).
This stone can serve as a 4th-to-2nd row foldback, even in situations, such as the one shown here, where it would not normally escape a 4th row ladder.
Note that Blue cannot yield, because then Red gets a 3rd row ladder, which the 4th row stone does escape.
Consider red stones at a3 and a5 (or the equivalent cells on the opposite side of the board).
Red can use this to get a foldback from the 4th to the 2nd row:
If Blue plays somewhere other than 4 or 6 (in either order), Red connects outright.
Moreover, Red can also use this to get a foldback from the 5th to the 3rd row:
The same two stones can even be used for a 6th-to-4th row foldback:
We say that a ladder carries a foldback threat if getting a foldback would allow the attacker to connect. Even if the attacker cannot actually get the foldback, the mere threat of a foldback can be an advantage for the attacker. An example is the 4th-to-6th row switchback using A4, which only works in the presence of a foldback threat.