Leach telltales, I'm sure there are a lot of opinions out there, for what it's worth here's my simplified explanation:

If leach telltales are flowing straight back it indicates that you have attached flow across the back side of the sail and that flow extends all the way to the leach. If you are sailing in conditions where you could use additional power, flowing leach telltales indicate that you can add power by changing to a fuller shape.

If the leach telltales are being sucked back behind the sail it indicates that flow separation (stall) has occured at some point before the the flow reaches the leach. More power and less drag will result if the sail is flattened.

Maximium power is derived when the leach tell tales are just on the virge of being sucked behind the sail and flowing. In overpowering conditions it is important to reduce drag and the sail should flattened to, or beyond, the point where the leach telltales flow straight back.

In short, I use luff telltales to drive by and leach telltales to tell me how to adjust sail shape.