SPS was designed within the army program "Grach" to develop weapons for enhanced 9x21 mm ammunition. It was rejected by army, but immediately picked up by law-enforcement agencies like FSB, as a replacement for APS pistols.
In the late 1990s, however, SPS was again picked up by army, and was modified - handle was redesigned for better ergonomy. Supposedly, it is currently used by FSB, FSO and SpetsNaz.
Technically, SPS is recoil-operated double-action pistol. It has two safety buttons. One is placed on the back of handle, and is switched off, when pistol is properly held in hand. Other is on the trigger, and is switched off priorly to shot, during trigger pull.
SPS uses powerful 9x21 mm ammunition, capable of defeating class 3 bulletproof vest (30 layers of kevlar + 2.8 mm titan plate) at 50 meters.