Bird Opening - 4/5..e6 Classical Variation
Here Black uses a reversed Rubinstein variation approach from the Nimzo Indian. While I frequently mention a reversed Nimzo Indian, there is a strong optical similarity, but to be accurate White is playing a reversed Dutch. The fact that f4 is played before Nf3 gives White nice control of the e5-square, which is not the case in a pure reversed Nimzo Indian move order. In a reversed Nimzo Indian move order, White would have to play Ne5, f4, and then Nf3, three tempi to get something similar to the above diagram, while in the Bird move order, White gets control of e5-square with only two tempi: f4 and Nf3. This is a big difference, helping White to get an edge.
Good Chess! Keith