The two telltales below the top one tell us about the overall shape of the sail. When the top telltale streams steadily aft but the second one is stalling, your main is either too full in the middle and /or too flat in the top. Bending the mid-section of the mast normally helps.The main sheet is the most important trimming device on your boat, and the top leech tail is the best indicator of the main sheet tension. The required tension on the main sheet depends of the boat you have; use the top leech tail when you are looking for the right trim in different condition. In less than 5 knots of wind adjust the sheet so that the upper tail is visible only sometimes.