that bar under the boom is called the rotator bar. Because your mast is in a tear-drop shape, you gain better sail shape if you rotate the mast according to the wind and wind direction you are sailing in. Typically, for upwind sailing, you want the mast rotated enough that the rotator bar is pointing at the daggerboard. This induces a nice smooth leading edge entry to the back-side of the sail for smoother (and more efficient/powerful airflow). For downwind sailing, you want the mast at 90 degrees so the rotator is pointed down the beam...or possibly a little more than that. Most non-spinnaker boats will use a positive mast rotator in the form of additional lines near the base of the mast to force the mast to rotate to that 90 degrees.