post exchange is when people agree to join and post so many threads or topics on each others sites.
the idea is that it brings new members and makes your site look busy which is great if your site hits a quiet patch.
for e.g ....
you can request something like a 5 for 5, 10/10 or 20/20 post exchange with the option to keep the posts on going if you actually like the site your exchangeing with.
how it usually works i have found on other forum promotion sites is that people request post exchanges, they submit their own rules and post limits, then if someone agrees to do the exchange they join each others sites and fill out that commitment and then either pm the person or announce under their request that they have filled that exchange.
it can work quite well and often people do stay on afterwards, the beauty of post exchange is that you can exchange with sites that arnt in your clique which helps get your site seen in many places by adding your site link to your signiture.
i hope this makes sense alpha

xxx