it is silly, stupid and pointless but it works and is fairly easy to fly.
1) toggles all jet engines (use when preparing to launch)
2) changes rapier engine mode (use when your apoapsis time starts decreasing at about 25 km)
3)toggles monopropellant engines ( use when in space to circularize and deorbit)
4)decouples big landing gear (use after takeoff)