When the airstream is unstable, the wind tends to oscillate (vary in direction) around a mean wind direction (the median). This typically happens when the wind is blowing from over the land, through city streets and over high buildings. The mean wind direction is called the median, while the wind that turns with you is lifting and the wind that turns against you is heading.
When you are tacking correctly in a header, you are in phase. If you are tacking when being lifted, you get out of phase.
Most of us know that you should tack when the wind is heading. The question is when to tack as the wind starts to head - immediately, when the wind heads 5 degrees, 10 degrees, or is it perhaps best to wait until the wind has gone all to way to one side or the other?
The answer is you should tack as the header passes the median. Why is this? It's quite simple really: that way you are always sailing on the tack that takes you closer to the windward mark. You end up sailing a shorter course than in any other way.
So, the key is to find out the wind median (the mean wind direction) before the race. That of course is much more difficult in the real world than in our little simulation. David Dellenbaugh explains how-to in the link list below:
