The aim system doesn't generate very much movement of it's own, instead it corrects your own movement. Simple example, if moving crosshairs towards a target but too low, it will adjust movement up slightly. If not moving the mouse at all, it will do nothing. When directly over a target, it tries not to do too much either (i.e. lock on) until you start firing, then it will be more aggressive.
In short, it's very much a soft aim assist which isn't something everyone likes, but which I like personally.