Punishing the 3.c4 Nb6 4.c5 overextension

With 3.c4 and 4.c5 White chases the knight, but the c5-pawn is left without support: ...e6 and ...Bxc5 win a pawn.

1.e4 Nf6 2.e5 Nd5 3.c4 Nb6 4.c5 Nd5 5.Bc4 e6 6.Nf3 Bxc5

Move by move

1.e4 – White occupies the centre and opens diagonals for the queen and the f1-bishop.
1...Nf6 – The Alekhine Defence: the knight attacks e4 and invites the pawn forward, planning to attack the advanced white pawns later.
2.e5 – White accepts the challenge: the pawn chases the knight and gains space.
2...Nd5 – The knight centralises on d5, from where it eyes c3, b4, e3 and f4.
3.c4 – White chases the knight at once with c4, before d4: an ambitious move order.
3...Nb6 – The knight retreats to b6, where it attacks c4 and invites the pawn to advance further.
4.c5 – A premature push: the c5-pawn is not supported by any pawn and the d5-square is free again for the knight.
4...Nd5 – The knight returns to the strong d5-square, where no pawn can chase it any more.
5.Bc4 – The bishop attacks the d5-knight and develops towards f7.
5...e6 – The pawn defends the d5-knight and opens the f8-c5 diagonal: now the c5-pawn is attacked.
6.Nf3 – White develops but neglects the c5-pawn, which stays undefended.
6...Bxc5 – The bishop captures the c5-pawn and develops: Black is a pawn up with a strong knight on d5.

Black is a pawn up with a stable knight on d5; White has some development in return.

The d5-knight cannot be chased by pawns: it is Black’s best piece.
Complete development with ...O-O, ...Nc6 and ...d6 to undermine e5 and open the game for the bishops.
White will want d4 with tempo on the c5-bishop: retreat it to b6 or e7 and keep developing.
If White plays Bxd5, recapture with ...exd5: the extra pawn stays and the e-file opens.

After 4.c5 the c5-pawn lacks the support of b4 or d4: it is a target.