I think my question was actually answered well. I actually now get it.
The only questions i have now is:
If a semi automatic has an outer hammer. Can you cock it back yourself and then fire the gun or because the trigger does this for you, there is no need?
Also, someone mentioned about carrying a a double action with an outer hammer? Do semi automatic with an outer hammer have to have a safety so if in the "cocked" position, the trigger cant be pulled so easily?