Definitions: Pawn Promotion and Pawn Breakthrough

These two are almost closely enough related to be the same thing but I want to make some distinctions.

With pawn promotion, we can generally say that a pawn that has securely reached the 7th rank already has the value of the opponent's lowest value remaining piece - the piece that must be sacrificed to prevent the promotion. Considerable material can be sacrificed if it ensures the newly promoted Queen cannot be captured.

Scholtz vs Lorenz, 1964, correpondence (ECM position #798)
Black to play

Black makes a pawn avalanche by 1. ... Nxd5 2. exd5 e4 3. Nc4 ef3 4. Nd2 f2 5. Kd3 f3 promoting.

Pawn breakthrough I want to look at from the point of view of what pawns can and can't do when unassisted by other pieces.

Crippled Majority (Queen Side) and Tactical Breakthrough (King Side)

On the King side White can force promotion with 1. g6 hxg6 2. f6 a decoy, for if Black doesn't capture, 3. fxg7 followed by promotion, and after 2. ... gxf6, the h-pawn is free to run home.

The Queen side of the board illustrates what can't be done. Black's majority is crippled. Even if the Black pawns get to a5, b5, c5, c6 any attempt to break through will fail, provided White is not forced into a capture as this will allow Black to create a candidate passed pawn which is promotable. Notice the a3, b2, c3 formation. If Black ultimately plays ... bxa3 or ... bxc3, White recaptures and Black's a-pawn and two c-pawns are blockaded. This formation occurs from the Ruy Lopez Exchange Variation, and represents White's compensation for Black's two Bishops; White tries to press with his King side majority, and can hold a draw in hand even if he loses his e-pawn.