It's not necessarily "slipping way" (or giving leeway). As you move faster through the water, you require less daggerboard to provide the resistance to keep the boat from sliding sideways.

Trying to explain this clearly; if your daggerboards are extended all the way down, the center of the sideways force they generate is further down in the water - roughly in the center of the board that is exposed below the hull. The further away this center of force is, the more tipping leverage is applied to the boat (wants to fly a hull sooner). When you are going faster, and since you don't need as much daggerboard to get traction in the water, you can raise the board and raise this center of force closer to the hull and the surface of the water. What you are after is to have enough daggerboard in the water to keep the boat going in a straight line but just barely enough so that the boat is much more controllable and not too tippy.

If you watch the wake coming off the stern of the boat, it will tell you how much, if any, side slip you have. the outside wake will roll over on itself when you slip sideways ... this tells you that you do not have enough daggerboard in the water. Try sailing with the daggerboards in different positions and watch the effect it has on the trail in the water...you will see.