brobru,
the HT mast bends to lee in a puff, but it also twists a little too. The net effect is that the sail flattens and dumps power at the top of the sail. The downhaul holds the mast in a backwards bend, particularly at the top - which combined with the correct sail cut, correct sheet tension, and mast rotation can flatten the whole sail.

Do this right and you are fast, do it wrong and you are slow.

Bill