Traffic trading is just that, a broad term for exchanging traffic.
Skimming is where a certain percentage of clicks are sent to a trade rather than the gallery. If you get the percentage right then you have a good balance of happy surfers (good productivity equals more clicks to FHGs which equals more $$$) and a decent sized site with good trades.
It's not always as easy as it sounds.
As I said above, it's going to be difficult to grow a site solely with hard links. TGP traffic is of a lower quality so you need a lot more of it to make a good income. Unless you have a lot of existing contacts to trade hard links with then you'll probably end up with a 3k site making $5 a day.
