Updated: May 2, 2019
The most fundamental thing all sailors need to understand is the power source - the wind. Good sailors are constantly looking upwind for puffs and they always keep the direction of the wind in mind. Wind constantly shifts (sometimes dramatically) both in velocity (puffs or gusts) and in direction. It shifts to the right and left. It increase in strength (a puff) or it can decrease in strength (a lull). The direction of the wind almost always changes slightly when a puff finally reaches the boat or the boat sails into a lull. The boat can heel over quickly, or it can flatten quickly depending on the change in wind direction and intensity of the puff leaving new sailors wondering what's going on and why is the boat behaving badly? It's simply the change in force applied to the boat.
Let's talk about direction. There are several methods to understanding wind direction. One is looking into the wind and hearing the sound it makes in both of your ears. Try it! If you turn your head slightly to the left or right the sound of the wind disappears in one ear but if you are looking directly into the wind you will hear the wind equally in both ears. Another method is to look at the ripples in the water. Notice that they make an arc and that is because the wind is pushing the water slightly. You can also feel the wind on your skin change in direction and velocity. Finally, you can look at a wind indicator at the top of the mast. What you can't do is look at a weather forecast and say "aha, the wind will blow southwest all day". That might be the general direction but because the wind is changing all the time that's not what's affecting the boat.
Now let's talk about velocity changes - puffs or gusts. In Lake Tahoe especially, the mountains churn up the air causing down drafts and the valleys can channel wind in waves of new puffs. When these drafts hit the water they move downwind with more velocity and can fan out in several directions. The best way to spot a puff coming down towards you is to look upwind at the water. A gust causes the water to turn darker in color, or it can also make the water more brilliant because of the sun reflection on the waves. Sustained puffs will make more waves cause a change in color and you can see these dark patches of water moving downwind towards you. If you are high up on a mountain, you can easily see these dark patches of water move downwind.
Good sailing teams have crew who can look upwind and tell the skipper if a puff is about to hit so the skipper can prepare for the boat to heel over. They can tell if one side of the course is windier than the other and if you're racing, that's the way to go because you want more wind to make the boat go faster. The power of the wind is going to make your boat go faster than anything else. You can have the fastest boat, the best sails, the cleanest bottom, instruments with polars, you might be great at steering, but if you are not in the stronger breeze you are going to be slower than a boat with old sails and a dirty bottom that is in the stronger breeze. When it comes to sailing, whether you are cruising or racing, understanding puffs, lulls and change in direction is the foundation for becoming a good sailor.