I've never understood why a crossover is needed in some situations but not others. If someone reading knows why can you enlighten me?

Actually a crossover is needed in all cases, so that the transmit wire of one device is the receive wire of the other device, and vice-versa.

The trick is that most hubs already have all of their ports crossed over. All except the uplink port which is designed to daisychain into another hub.

So you use a straight cable when plugging into a hub, or into any port that's already been crossed over. And a crossover cable into any port that hasn't been crossed over at the port.

Probably makes it clear as mud for you... Sorry...