No indicator has been developed that approaches the sensitivity and effectiveness of telltales along the luff of the jib. These yarns or “ticklers” monitor the flow of wind across the sail.
Telltales are used for fine tuning your genoa sheet trim and to fine-tune the course you are steering. Telltales are only an aid when the sail has wind flow across both sides, i.e., when sailing angles between beating and beam reaching. When sailing lower than a beam reach, the sail is catching wind instead of working like an airfoil.