blingice: Quite an amazing game. Here's some of my comments:
Youngworth breaks from book moves on his 17th move: Opening Explorer. However, before that, he was sticking to some of the most common moves of theory.
20. Rg7!!: Black can't move 20..Bxg7 because the follow up is 21. fxg7 Rg8 22. Qxh7 and the rook will be lost.
21. Nf5!!: black can't move 21..exf5 because white responds 22. exf5 and the e file is open for white's rook.
22. e5!: remove's the protection of white's queen from the 7th rank. If black didn't take the e pawn, then DeFirmian could possibly play a very invasive knight move like Nd6. (I may be incorrect about that plan.)
25..Rxb2+!!: white can't accept because the queen can get perpetual check. (Am I correct about that?)
36..Ke4: 36..Kxf6?? is bad because it becomes 37. Qf8+ Kg5 38. Qg7+ and then white's rook may come to help with checkmate.