This add on board solders on top of the MS3 daughter board and gives you two channels of knock input that the MS3 can use to retard spark. Using this module will give you a much more sophisticated knock detection circuit than on any previous MegaSquirt knock detection. Features include a programmable bandpass filter, levels that change with RPM, the ability to assign different cylinders to different sensors, and different gain levels for each cylinder. See the MS3 manual for details.