1gb Ethernet works in full duplex and the maximum theoretical throughput is 2Gbps. Every pair gives 250 mbps in both directions. How it can work in full duplex on the one pair?
Two waves can propagate down the same pair in opposite directions without interfering with each other. With the right circuitry (known as a "hybrid")you can launch a wave and receive a wave at the same time. In principle with perfect components you can design a circuit that will keep the launched waveform and received waveform completely seperate.
In reality no components (including the cable and connectors) are perfect, especially at high frequencies so gigabit Ethernet can't simply rely on an Analog hybrid circuit. Instead the devices must perform active equalisation and echo cancellation to receive a usable signal.