Modern CZ's use a rebounding hammer and an inertial firing pin. This is another safety built into the pistol's design. The idea is that the firing pin has some mass to it, and it needs the full swing of the hammer from its fully retracted position to accelerate it enough to strike the cartridge's primer with enough force to cause ignition. Going off half cocked won't whack the firing pin hard enough to move the firing pin all the way forward to strike the primer, such as when you are pulling the hammer back with your thumb and the hammer slips before it reaches either the half cock sear or the main sear.
The rebounding hammer prevents the hammer itself from simply pressing the firing pin against the primer with enough pressure to ignite it. The hammer is mechanically prevented from moving all the way forward if the trigger hasn't been released and pressed again.
Another safety is the firing pin block, which is raised by trigger movement before the trigger can move the sear out of engagement with the hammer. The firing pin block prevents the hammer from moving the firing pin forward in the event that the pistol drops onto pavement, breaking the sear and moving the hammer into contact with the firing pin.
And there are yet more safeties designed into the pistol, but you can see those....
Scott
