I don't know why but the way "Aguila Blanca" wrote it, i could actually picture everything perfectly... Im gonna type it as i took it and hopefully now i get it.
So, basically, every type of semi automatic needs to be chambered, but with a SA only, after every round is fired, the "hammer" is cocked by the slide and the trigger only releases the hammer to fire the round.
with SA/DA.. it works the same way as above, but if the hammer is down, the trigger cocks it and releases it rather than me having to cock the hammer manually or by the slide.
DA only, racking it only chambers the round and the trigger does everything.
How did I do for this test? :-)