A computer-based method and system for controlling the mortgage rate charged to a mortgagee as a prevailing mortgage rate drops. The rotary latch receives a locking part and can be transferred either manually or by means of a motor between an open position, a preliminary catch position and a main catch position. The at least one buffer device is mounted in the piston head and has a housing, a valve stem and a buffer spring. The light-emitting method includes detecting an air pressure of the light-emitting device and controlling the emitting member to emit light with color and luminance value according to the detected air pressure. The airfoils may be provided individually or as a set.