No matter what the second player does now, his first move is the third edge of a box, so he gives up that box, and when that one is complete, the other box on that side can be taken. So first player gets 2 boxes. First player's final move then gives up the other two. This response is the two remaining middle lines. No matter what the second player does now, his first move is the third edge of a box, so he gives up that box.
First player responds by playing the other middle line immediately.
This is the third side of a box, so his turn ends immediately. Second player claims the box, but his next move gives up another box, and we see the alternation of boxes again. This response is any two lines in the same cell. Obviously, this gives up that cell. First player takes it and then plays the middle line on the other side.