Yeah, i did, and i though I understood it, guess not. I thought an SA/DA gun (at least, according to the video i saw) has its hammer cocked back by the pressure generated when a bullet is fired. SO if the hammer is decocked, wouldn't it not really do anything until you cocked it back, fired, upon which the hammer would be cocked back again?
ok...when you fire a da/sa gun several things happen and/or you can do them different ways and also depends on the type of handun.
in SA/DA semi, you rack the slide to chamber a round and the racking of the slide also cocks the hammer. With the hammer back, the gun is in SA mode, so the trigger pull is going to be lighter. After it fires, the slide comes back and re-cocks the hammer and puts the gun back into SA mode.
If you chamber a round and lower the hammer, the gun is now in DA mode. Pulling the trigger also cocks the hammer and releases it at the same time. This makes the trigger pull much heavier. The gun fires and the slide comes back and cocks the hammer. so you're now in SA mode again. to put it back into DA mode, you have to lower the hammer, either with a decocker or manually by holding the hammer and pulling the trigger, slowly letting the hammer come down.
On a SA/DA revolver, you can pull the hammer back before each shot which rotates the cylinder and cocks the hammer. After you fire, it will be in DA mode. You can then pull the hammer back again or just pull the trigger which will cock the hammer, rotate the cylinder, and release the hammer to fire. So on a revolver, it kind of works the opposite way after each shot.
In a SA only, You chamber a round which cocks the hammer. Again, SA mode has a much lighter trigger pull since you are only releasing the hammer, not cocking it at the same time. Slide comes back and cocks the hammer again and you're back in SA mode.
SA only revolver you just have to pull the hammer back after each shot. pulling the trigger while the hammer is down will do nothing.
If you lower the hammer on a SA only semi and pull the trigger, nothing will happen as the hammer is not cocked. You have to either rack the slide or cock the hammer after each shot. On a SA only semi, like the 1911, the slide coming back after each shot cocks the trigger.
In DA only, you chamber the round and the hammer is not cocked or may be only partially cocked. pulling the trigger cocks the hammer and releases it, firing the round. the hammer then is back to it's original position, either uncocked or partially cocked. DA only guns probably don't have a traditional hammer or a visible hammer, or a hammer at all (striker fired pistols).
On DA only revolver, the hammer will be lowered after each shot and pulling the trigger rotates the cylinder and cocks and releases the hammer.
to further complicate things, some guns have a magazine disconnect. If the magazine is not inserted into the gun (or not inserted properly), it will not fire, even if the hammer is cocked and there is a round in the chamber.
and as a safety tip, never rely on a mechanical safety. mechanical parts can and will fail. this is 1 reason why you should always point the gun in a safe direction.