How does it work?

On standard engine mounts, the outboard must be mounted low enough as not to cause cavitation when running into chop or executing turns. In calm water on a straight course, it could be mounted higher, but when it's bolted tightly to the transom, moving it up or down obviously can't be accomplished. That's where a jackplate comes in. Tilting up the engine to run across shallow water puts the cavitation plate in front of the propeller, actually encouraging cavitation. Lifting the engine vertically keeps the plate above the propeller and aligned with the movement of the boat. Thrust channels directly parallel to the surface of the water - the most efficient direction. A tilted engine throws a rooster tail of water up into the air. If you throw water up, you waste energy. Plus, Newton's Third Law of Motion says that thrust upward must equal thrust downward. If you're in shallow water, you don't want your boat to be pushed down, especially at the stern. An engine tilted bow-up at a high angle to go across shallow water will raise the bow and lower the stern, making the draft of your boat deeper. A boat equipped with a jackplate can avoid all those negative consequences of tilting the engine. By simply raising the engine up with the jackplate until it is just on the verge of cavitation, you can run and start up in far shallower depths than you ever imagined.