It was another incredible day of racing today. The breeze of 15 to 20 knots, pretty flat water, 15-degree oscillations, beautiful emerald water: it's why we're here. These conditions put a real premium on knowing what phase of the shift you are on. There was a slight favoring of the left, as there often is, but mostly you have to stay in phase. Therefore you need a firm grasp of the median true wind direction. Use you warm up time before the start to establish the total range and the average wind direction, and state those numbers explicitly. I like to have a little discussion with the helm and main trimmer about these numbers, so everybody is on the same page. This can make racecourse decisions much easier.
On smaller boats without instruments you can use the compass in the same way. You have to be a little bit careful to not confuse puffs and lulls with shifts, because the compass does not know the difference. A puff will usually feel like a lift initially. Some dialogue from the driver can help the tactician sniff out the real shifts. Even on boats with full instrumentation, the compass can be more accurate than the TWD, because the TWD is only from one point, and the heading considers the whole sail plan.
So let's say we have a well-established set of numbers, and we tack on each major shift. Easy right? There are two additional considerations. Firstly, there may be a macro trend in the wind, or there could be a geographic factor to consider. For example, today there was a little bit more wind to the left, and more lefties than righties at the top of the beat. So you were served by taking neutral shifts on starboard, to play left of center. The second point is that simply playing the oscillations will not always bring you to the mark. There may be an unequal distribution of left and right shifts, or the course may be skewed. In this case the tactician has to tack earlier in the shift for the long tack, and later on the short tack. Another good reason to have it clearly in your mind what the TWD median is.