Regulating the Let Off distance correctly will almost magically improve on the overall responsiveness of your piano keyboard. It is a tough concept for layman to understand, so I won’t go through the theory behind it, but just the how-to. Firstly you got to know how to measure the let off distance. Depress the key that you going to regulate very very slowly. The hammer will travel towards the string, but because of the soft and slow depression of the key, the hammer will not hit the string, instead it will fall back to rest position at some distance away from the string. That distance is the let off distance that you are about to measure. Now use a steel rule (like the one in the video), put the steel rule on the adjacent hammer of the one you’re going to regulate. Measure the let off distance. The optimum let off distance for a Kawai upright piano is about 2mm in the treble, 2.5mm in the mids and 3mm in the bass. Use a regulating tool to turn the regulating screw counterclockwise to shorten the let off distance, vice versa.
When you’ve finished regulating the let off, your keyboard will be much more responsive than before. You have to keep in mind that it is easy to know how this is being done, but the responsiveness of the piano keyboard relies heavily with other factors. To regulate the keyboard to its optimum responsiveness needs lots of experiences and patience.