As full of luv stated, the swap with pot is for trading a trip you already have for a trip in open time. There is always a lot of behind the scenes churn that happens during pcs runs, especially the first couple on the 20th. So just because you don't see a trip in the pot does not mean it may not come available in the middle of the run. For example, a senior guy may get awarded a trip drop, that trip is now available for swap/white slip throughout the rest of the pcs run, even though it never shows in open time.

Swap with the pot also allows "wildcard swaps" This allows you to trade a trip you have for ANY trip that comes available. These are inputted as a * followed by the number of day trip you want. Example...to swap for any 4-day on the 15th you would input "*4 15may". Very useful if you need a specific day off, but don't want to use a drop or want to trade a shorter trip for a longer one. However, use these with caution as it will award ANY trip that comes open meeting the criteria even those you don't see in the middle of pcs runs.

Once again it is pretty easy once you look at the template and read through the help menu.