Things are already slightly surprising: Black first supports his d5-pawn with 2...c6, then he voluntarily exchanges away from the centre with the said pawn. If White could play e4 and Bxc4 without loss of time, Black’s plan would certainly be of doubtful value. But as a matter of fact this is precisely what White cannot manage. If he tries, for example, 5.e4, then 5...b5 defends the c4-pawn and shows that 2...c6 had a second purpose in addition to the protection of d5 – the preparation for ...b5.
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