White to play for a strong advantage
A
Critical position is defined as a precise moment during a game where a move alters the phase of the game. It can be a sharp change in evaluation of the position (as found by an engine such as Rybka, Fritz) but not necessarily: it could be for instance giving up a pawn in exchange of pressuring the other side even though the game evaluation is " chances for both sides", as defending is usually harder than attacking). In average, 2 to 4 critical positions occur per game.
For a more detailed discussion,
http://www.chesscafe.com/dvoretsky/dvoretsky.htmwhere the famous russian coach defines the notion almost scientifically or philosphically. Now in the position I above, Black just played 1..Ra7? and there is a two-moves sequences taking advantage of the awkard position of the black pieces as White did sacrifice a pawn earlier.
Eventually, the game continued with
2 Rc1 Ba5
3 Bf4 (
threatening Bb8) 3..Nxc5
4 Qd4!
Position IIBlack to play and stay in the game
Black is now two pawns ahead but is under severe pressure on the c column while the Bishop in a5 is hanging. This is absolutely critical to find the right continuation (which I failed to do by the way!). This is a very difficult position with hidden tactical resources for both sides. If only we could get IM Dvorestky to analyze it properly :)
Anyway,
there are two reasonable attempts here for Black. Calculation required either way!
Calculate well !
