They make the air flow over the sails visable. The tell tails will follow the direction of flow. You want the flow running generally fore to aft. If the air flow is disturbed, you can see it in the tell tails.

You can make adjustments in helm and or sail to correct the flow for best performance.

Basic adjustments are:



Hobie Cat recommends the following placements:



Leech tails are used as fine adjustment of the main and should flow aft then hook to the back side and alternate between those two. Hooking all the time is over sheeted. Flowing aft all the time is generally under sheeted.