You send in enough flies for each member of the swap (plus an extra or two for the swapmeister, for all their hard work).
So, if there are 10 people signed up for the swap, send in 10-12 of your pattern.
Once the swapmeister receives the flies from each participant, he/she will divide up the flies evenly (so each participant gets an example of each of the other flies, INCLUDING their own fly back), and returns the re-supplied boxes back to their respective owner.
Don't worry about getting your own fly back...using the Crayfish Swap as an example, you will now have a single box with 10 different patterns to review/use/copy. Its really nice to have all those examples (including yours) in one place. If/when you decide to use the flies, you just grab the box, and can easily compare the relative actions, colors, features, and success of the other tiers creations against your own. GREAT field test!