Well here is how the mechanism works:
1. after you insert the magazine you have to rack the slide to load the first round in the chamber
2. this will result in the following:
a. round in chamber
b. the hammer is cocked
c. the gun now is in S/A
3. At this stage, if you do not want to fire, you can lower the hammer (either manually or by using the decocker lever if you buy the one with decocker) - please note that lowering the hammer manually requires precautions and its own safety procedure
4. After the hammer is decoked, now the gun is in D/A. If you pull the trigger at this stage, the trigger pull would be heavy because the trigger pull is required to raise the hammer and then break the hammer release to fire the first round (just like a revolver D/A)
5. once the first round is fired in DA, then the slide will automatically rack and load the next round into the chamber. Now the hammer is cocked and the gun is in S/A mode, with a lighter trigger pull - and so on..
hope this will help.