How does “bidirectional” transmission on gigabit ethernet work 2?
This is a lot easier to understand if you're familiar with the rope wave metaphor: 

In (a) the hand has waved up and down and a wave has started to travel along the rope. In (b) it's moved again and sent a second wave, as if corresponding to sending (+1, +1). You could imagine moving the hand down to send a -1 wave.

Multiple impulses can and will be in flight along the wire at the same time. Including reflections of your transmission from the other end.