Originally Posted by bengal-stripe
You button it into the waistband of your trousers, to prevent the shirt front from rising up and billowing out.
(The flap goes down, picture shows it in the 'resting position'.)
Used to be a common feature on day shirts (all the Duke of Windsor's shirts had it), you still find it on evening shirts.
You know, that's what I first thought, but the button holes are too small to accommodate anything...