The" attitude" of a boat in the water can be controlled to a large extent by" trimming" the outdrive/propeller unit in relation to the stern of the boat. As the drive is trimmed up,the prop exerts more force downward,causing the bow to lift out of the water. As the drive is trimmed down,the force is exerted more in line with the boat hull,resulting in the bow riding lower in the water. Depending on a given trim angle,you can see increased or decreased speed,fuel economy,comfort and control at a given prop RPM. The trim gauge connects to a resistance based sender in the pivot of the outdrive. This functions much like the sender in a gas tank,with the gauge measuring resistance of the sender from totally down/in to totally up/out. This allows the operator to adjust for conditions and load quickly and easily.