5th.

nice

Yes, that's what I meant : forks hidden in variations which didn't actually happen during the game, but are important to evaluate some positions

And yes, 32...axb4 33.cxb4 Rxb4+? 34.Nc6+ is one of them

I think by "hidden" forks he means forks that didn't happen, but that you must see in variations to understand the game.  For example: 32...Kd6. You need to see that black cannot win a pawn by 32...axb4 cxb4 Rxb4 due to Nc6+ forking the rook.

Good folks are 13. Nxd7 ..,  23. Ne5 instead of Bf1 and 32. Nc6 other there are many more. the Knight is a natural folker.