same way as air flows across the sail. It creates a sideways force to windward that resists the opposite force on the sail. The two sideways forces cancel each other out, leaving a forward force which drives the boat. The force on the sails acts roughly at right angles to the boom. With sails in tight (when sailing close to the wind) the force acts mainly in a sideways direction. More sideways resistance is therefore required from the daggerboard. This is why most dinghies and some multihulls have a lifting centreboard or daggerboard, so that the area under the boat can be adjusted to suit the point of sailing.
The sails are sheeted in when sailing upwind. There are two basic ways to understand your performance. There is mechanical feedback from items such as tell tales, compass, burgees, and the luff (front) of the sail. There is also a more intuitive response, where you sense the ‘feel’ of the boat. The best option is to combine the two. The ‘feel’ will come with experience.