Notes by Dave:
I subsequently ran the game through Fritz, which came up with the combination I missed after Jim’s move 11. Rf3, ie 11….exf4 12. exf4 Nxf4! 13. Rxf4 Bxg5 winning a pawn with tempo, as White has to retreat his Rook. Fritz also commented “and Black prevails” giving the position a “rating” of minus 2.86, which I will come back to.
As Fritz said, the essence of the combination is “removal of the guard.”
This technique merits close study. I have spent many man-hours studying
tactics over the last couple of years and only one book has mentioned
“removal of the guard” so I have hardly looked at it, until now.
Lesson 3: in the absence of books and software, set up removal of the guard positions and try them on someone else, as setting positions up is an excellent way to practice (“the best way to learn something is to teach it”).
I said above that Fritz gave the position I would have obtained if I’d spotted the combination a “score” of -2.86. Fritz’s “scores” are rated in “Pawns”, so “-2.86” means it rates the position as Black being the equivalent of nearly three Pawns ahead, hence its comment “…and Black prevails.” Had the score been 1.3 it would mean that it rated White as a bit better than a Pawn ahead. It is a measure of the distance I have to travel that a) I can see that Black would be a Pawn ahead, because I would have won a Pawn, but I have no idea how it reckons up the other 1.86; and b) I can’t see for the life of me what I should do next, had I managed to play the combination.
Lesson 4: find a good player to show me how to start to analyse positions and make plans accordingly.