When underway, trim lets you to adjust the running attitude — the angle of the boat's bow — by trimming in (to lower the bow) or out (to raise the bow) to achieve the smoothest and most efficient ride. In a head sea, for example, trimming in lowers the bow, allowing it to knife through waves. In a calm sea, you might want to trim out to raise the bow for less wetted surface and greater efficiency. However, too much positive trim can lead to "porpoising," with the bow rising and falling in a disconcerting oscillation. Trimming in brings this under control.