well,when you press back button , the current app goes in background. It means it goes in "cache" mode. Here ,current state of app is saved/persisted by Android OS. Hence you dont need to use ATK. Also when this is done , app does not keep on running. just its current state is saved.
When you open that app again , its started from where it was left in most of the cases. this way you save a lot of CPU cycles and battery. cauz starting and closing app completely consumes more time (cpu cycles and hence battery) than restoring its state.
you dont need ATK.thats it. or let me put it in other words, Just use ATK when you feel that phone is lagging really bad (which is very very rare on 2.3.3 and S2) . dont keep ATK running 24x7. on my SGS , that the only app which drains battery as it as to monitor everything for its work.