What Use Is An F-call?

What use is an F-call? In passing a few weeks ago I mentioned listening 10-up. It's also a slogan I have on a t-shirt, it says: "I'm not ignoring you, I'm listening 10 up." So what does that mean and what do you do when a station tells you that they are listening "up", or "down"? If you're a DX station and you've got a desirable call, it's likely that you'll generate a pile-up, that is, lots of different stations all calling at the same time, trying to get the attention of the single DX station. As more and more stations join in the fray, the remote station will get drowned out by eager hunters who try to call early, or try to call late in an attempt to get the attention of the DX station. The impact of this is cumulative. Over time, the DX station will get buried entirely in spurious transmissions, so making a contact becomes harder and harder, sometimes impossible. I've talked about the rhythm of a contact. If it's all working as expected, the rhythm will help you synchro