well, since im here now i can give a quick answer
anyone can host a swap - they just check the interest on the board and let it go
they then give a different "weight" to each item - so if you are making a frame you might need to make two - but if you are doing a shaker box or a piecing you only need to do one
so the person posts the theme - as an example magic kingdom - each person signs up for what they want to make and what attraction they are basing that on - no two people will get the same attraction - and rarely are more than one person making the same type of item - you then make enough for everyone in the group, send your pieces to the person leading the swap and then you get a wonderful package in the mail with all your pieces!
if you go to the top of our group there is a thread there called something like "swap calendar" and in there are links to pictures from some of the previous swaps!
im sure later in the day you will get a better explanation (i havent had coffee yet!)
have fun!