The Fork applies to all stages of the game. Here's an example in a 6x6 position.
Problem 30 shows a fork as the final pointe of a stunning combination Demian performed against me at the age of eleven. He had been playing Bushka for over a year at the time. In the actual game, I played 5. f4x6, and the fork never materialized. Instead I lost my second man in a none less subtle matter.