It is essentially a "low gear" for the 31. It reduces pressure on the clutch plate by letting a small portion of the hydraulic fluid go into a bypass situation. This allows the clutch to slip in a controlled manner, reduce the speed that the prop is rotating at, and therefore reduce the boat's speed by a few knots.
A very rough analogy would be driving your stick shift car with the clutch partially depressed so that the clutch plate is slipping; you go slower. (in a car, this would result in a burnt-out clutch very quickly, but marine transmission are built differently and can handle this)
lornec wrote:Excuse my uninformed self , but what is a $2000 electronic trolling valve? Its listed as an option on the new 31