Simple english: if two computers are connected by a full duplex ethernet link, then they should be able to carry out two conversations with each other simultaneously. For example, imagine two computers named A and B with a 100Mb/s full-duplex connection linking them both. A starts "talking" at 100Mb/s and B "listens". B also starts "talking" at 100Mb/s and A "listens". The total data moving up and down the link 200Mb/s. That's full duplex, baby!
Only, in real life you don't get the full 100Mb/s in either direction. On my PC, I managed to get 91Mb/s in one direction and 61Mb/s in the other direction. If I stopped the 91Mb/s conversation (call it X), the 61Mb/s conversation (call it Y) would quickly use up the extra bandwidth, becoming a 91Mb/s conversation itself. As soon as I restarted X, it reclaimed its original 91Mb/s, and Y returned to its original 61Mb/s. Freaky.