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Passed Pawn Extensions are triggered after a passed pawn is established on the seventh or sometimes sixth rank, that is after a pawn push or captures with a pawn. The idea is to avoid horizon effects regarding a possible promotion, and to determine whether the stop- or promotion square is or may blocked, defended or supported. Clearly, this extension heavily interacts with the evaluation of passed pawns, and may also restricted to certain node types and distance to horizon. Some programs also extend promotions itself and not only if there starts a threat to promote.

Colin Frayn

Colin Frayn on Pawn Push Extensions ^[1]:

If a pawn is pushed to the 7th rank, or promoted, then extend.
This helps spot winning tactics in the endgame.
Not so useful if your endgame analysis function is good enough at handling passed pawns.