"So it turns a Glock into an open bolt SMG?"
Not really; the way the FSSG and similar Glock conversions work is by adding an angled plate onto the front of the backplate, that engages the rear part of the trigger bar as the slide moves forward into battery. Just as the barrel raises back into battery (the locked position, where it's safe to fire a shot), that angled plate pushes the rear of the trigger bar down just far enough to let the striker slip off of the trigger bar, firing a shot.
Not really; the way the FSSG and similar Glock conversions work is by adding an angled plate onto the front of the backplate, that engages the rear part of the trigger bar as the slide moves forward into battery. Just as the barrel raises back into battery (the locked position, where it's safe to fire a shot), that angled plate pushes the rear of the trigger bar down just far enough to let the striker slip off of the trigger bar, firing a shot.