With Variable Power recoil springs, the space varies between coils with less space between coils at one end and more space between coils at the other end. They are beneficial with compensated handguns and when using light target loads where less recoil energy is available. Wolff Recoil springs fit over your handgun's guide rod and control how the slide functions. In general, heavy loads need heavy springs and lighter loads need lighter springs. There is no hard and fast rule as to how much "spring" a handgun needs. Too light of a spring and the handgun opens and closes early and hard, increasing the felt recoil and battering the frame. Too heavy of a spring, the slide is hard to open and may not cycle completely, jamming the firearm. Includes an extra power firing pin spring. Installation requires disassembly and reassembly of the slide.