4.Nf3 attacks the undefended Black Pawn at d4. The only way to defend this Pawn is by playing c5, which would restrict Black dark-squared Bishop and place Black behind in development. But White’s thinking is flawed, as there is an intermediate move which would allow the Bishop to develop with a tempo.
4...Bb4+ forcing 5.Nbd2 because 5.Bd2 Bxd2 6.Qxd2 c5 is strong enough for Black. Now we can play 5...c5 and exchange the bishop at a time of our choosing.
In my study I often write down further analysis - possible continuations - that I don't usually include in this blog, but due to the short nature of this one I have this time!
- To develop unhindered White may have to choose to play 6.e3 and exchange his last remaining centre pawn, for Black’s doubled Pawn.
- Another option for White is to play 6.a3, but after 6…Ba5 7.b4 cxb4 8.Nxd4 Nc6 and White has very little for his lost Pawn.