There are some openings where Black can play his third and fourth moves in any order. So there's the very common sequence 1. e4 e5 2. Nf3 Nc6 3. Bb5 a6 4. Ba4 Nf6 5. 0-0. It isn't possible to win a pawn by 4. Bxc6 dxc6 5. Nxe5 because the reply 5. .. Qd4 hits both the pawn on e4 and the Knight on e5 and if 6. Nf3, then 6. .. Qxe4 is check.

Contrast this with the sequence 1. e4 e5 2. Nf3 Nc6 3. Bb5 Nf6 4. 0-0 a6 . In this position 5. Bxc6 dxc6 6. Nxe5 just wins a pawn, since after 6. .. Qd4 7. Nf3 Qxe4 8. Re1 would win the queen. Other similar tactics are available against other attempts to regain the material so Black has just lost a pawn for little compensation.

Opening knowledge is sometimes equated to knowing what to do at move 15 in the Marshall. Arguably it's also knowing the main variations and tactics in the first few moves and when it's safe to vary the order.