The Endgame Tactician: Zugzwang & Steinitz's Rule
Simba writes, Thanks for posting [Karpov-Hort, Budapest 1973] and your commentary which is very informative. Got one question: What does 'spare tempi in zugzwang situations' mean when white moved g5 in move 35?
Zugzwang means it's a disadvantage to have the move.
In the above position, Black to move loses and White to move draws. Such positions are called zugzwang positions and they're very common in the endgame. It's astonishing the first time you see such a position, because in the opening it's an advantage to have the move!
Steinitz's Rule: Keep some pawns on their starting squares, because there they often provide reserve tempi in zugzwang positions.
With a-pawns onboard, the situation's more complicated.
In the first diagram, Black to move draws with 1...a6! a4 a5 and now it's White to move in our original zugzwang position. But he loses with 1...a5?? 2.a4 as now it's Black to move.
In the second diagram, Black to move draws with 1...a5! and now it's White to move in our original zugzwang position. But he loses with 1...a6?? 2.a5 as now it's Black to move.
In the third diagram, Black loses either way. 1...a5 2.a4! and it's Black to move in the original zugzwang position. 1...a6 2.a3! a5 3.a4 and again it's Black to move.
Not every endgame reaches a zugzwang position, but many do, and now you know why it's a good idea to leave some pawns on their starting squares.