APP Killer

Can someone explain to me how this works...tried a couple different apps and still not understanding. currently using astro file manager. are these programs supposed to automatically kill an app after it has been used? because ive killed an app then selected it from home screen and when finished using it, it shows up as running when i go look at running apps. may be a stupid question but would like to know. Also please let me know what apps are good for this.Thanks.