This article is on break moves, but Dan Heiman has already written the perfect break move essay at Chesscafe, see here - http://www.chesscafe.com/text/heisman17.pdf. Most of what I present can also be found there.
After you develop, or along side developing you will often want to break the pawn position to increase the mobility of your pieces.
Dan writes: "A 'break' move is a pawn move that 'breaks up' an opponent's (fixed) pawn chain by attacking the opponent's pawns with that pawn."
Break moves are not solely for the use of the defender, but they are definitely required more when defending. The attacker should often seek to advance a pawn alongside his most advanced pawn. This action can also be thought of as a break move, but is often done more to gain space and cramp the defender.
White would like to advance a pawn to either e4 or d4 the yellow squares. Black would love to break with c5 or e5 the green squares. As always with chess short term tactics need to be considered, c4 is a much stronger move than e4, a Black pawn on c4 can be recaptured while e4 is a gambit. Note that a true break move requires the pawn to be fixed otherwise the pawn may just be pushed past.
Lets look at another position: After 1.e4 e6, 2.d4 d5, 3.e5 we have the Advanced French
Black's break moves are c5 or f6. f6 may leave the king a little weak, so may be delayed until the king is safe. Nimzowitsch taught us to attack the base of a pawn chain, so c5 is very strong. If White captures then Bxc5 develops a piece and leaves White with a pawn on e5 that needs to be defended.
In Dan's article he presents the following position:
1.d4 Nf6 2.c4 g6 3.Nc3 Bg7 4.e4 d6 5.Be2 O-O 6.Nf3 e5 7.O-O (7.dxe5 dxe5 8.Qxd8 Rxd8 9.Nxe5 does not win a pawn because of 9…Nxe4 and if 10.Nxf7?? then 10…Bxc3+ wins a piece) 7…Nc6 8.d5 Ne7 - The mainline King's Indian
In this position what is Black’s best break move (hint: it is not legal – yet)?
Black wants to attack the base of White's pawn chain, so breaks on either b5 or f5. Remember though the pawn needs to be fixed to be a break move, so b5 is not an option. Therefore Black wants to play f5. If you look through a King's Indian book you will see this position and perhaps wonder why Black plays either Nh5, Nd7 or Ne8 on his next move, depending on White's move, and doesn't move the at home bishop on c8. Now you know the strategy of breaking, the knight moves don't look so strange. Some of White's ideas in this opening are to stop Black's best knight move, so you will see lines beginning with 9.Ne1, or 9.Nd2, both prevent 9...Nh5 and allow f3 to support the e4 pawn.
Perhaps White should be trying to advance his c-pawn to c5 so he has a pawn alongside his most advanced pawn, but c5 allows the pawn to be captured, which cedes some advantage to Black. 9.b4 preparing c5 is one plan in this position and makes perfect strategic sense.
Note that in opening play, theory sometimes finds flaws in plans that seem perfectly strategically sound. That is because these ideas have been tested in thousands of games and tactical flaws discovered.
Please read Dan's article it is available for free and is brilliant!