Reddit mentions of Principles of Yacht Design, 3rd Edition

Tightening your cunningham flattens your sail, and a very direct consequence of this shape is that it opens your leech. This is generally a Good Thing^tm when you are in very light wind conditions.

If you have leech tell tales, you can look at them to see how your air is flowing out of your sail.

In general, in very light wind conditions (meaning the wind is likely more laminar than turbulent), a very draughty sail will stall easier.

Obviously this is all subject to experimental assessment, but the take home message should be this:

• light wind = laminar flow = easier stall
  
• more draught = more power but easier to stall
  
  Your goal should be to maintain flow above all else, and vary the parameters in that enveloppe. So try adding draught (loosening your cunningham) until you see your leech tales starting to curl back...
  
  (wave conditions also come into play when you realize that bashing into chop will stall the sail)...
  
  
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  If you are really interested in the dynamics of how and why this happens, get a copy of Principles of Yacht design by Larsson and Eliasson, and look at the foils section for an excellent description of this. Also, you can look at NACA foil designs and how they work. It is very well explained in that book. You will be surprised how much information is actually codified proper in engineering/scientific terms (e.g. draught position on the sail)...
  
  PS. Sailing is a very dynamic field. You are best served by learning and understanding our current state of knowledge as opposed to taking blanket statements a-la "never do that".