Hidden Passed Pawn
The Hidden Passed Pawn is an evaluation term used in Crafty, in the terminology proposed by Hans Kmoch it is referred to as a Sneaker  . It is a rammed pawn already on the opponent side of the board without further sentries, which may become a passer through a sacrificial combination usually by a faker, which possibly allows the opponent to establish a passer as well, though at least two ranks less advanced and losing the pawn race assuming both unstoppable.
Suppose that white has pawns on a6 and b5, and black on a7. At first glance we have a backward pawn on b5, which is bad for white. However, after playing b5-b6, white gets a passed pawn either there or on a6. For that reason the pawn on a6 should be termed "a hidden passed pawn" and awarded a bonus if "unstoppable".
Fakers on b5 and g4 make a6 and h5 Sneakers
Comparison with Candidates
If we put the sneakers of the above position one rank back so that they become helpers of the now neighbored candidates, the usual idea is to advance the candidates instead of the helpers. The lonesome sentry, forced to capture, may be recaptured to establish an exclusive passer on the candidate's file. Alternatively instead of the recapture, the helper may advance to become a passer as well, which yields to a transposition of the hidden passed pawn idea. This may be stronger if the pawn becomes an unstoppable passer leaving the opponent king outside its square, while the candidate could be caught.
a5 or b5?
- Hans Kmoch (1959, 1990). Pawn Power in Chess. New York: Dover, 1990. Previous ed.: New York: McKay, 1959. ISBN 0-486-26486-6
- Pawn Power in Chess by Hans Kmoch - Glossary of Terms - Chess Forums - Chess.com