Those little bits of red and blue yarn or ribbon on your sails can tell you a lot about how effectively you are steering and trimming. This great article from WB-Sails is one of the most informative I’ve ever run across on the subject. The animated diagrams are just great! If you feel a little fuzzy on why the telltales on your sails do what they do, it’s well worth a read.
If you are sailing too low, or if the sail is sheeted in too hard, a separation bubble forms on the leeward side. This is very harmul to the performance and should be avoided at all times.
Independently of the luff separation bubble, the airflow may get detached from the sail just before the leech on the leeward side. The eddy formed like this makes the leech telltale disappear behind the sail. For the sake of clarity, the size of the separation bubbles is exaggerated in the drawing.
Thanks to LiveSailDie for the link.
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Very good and clear drawings and explanation.