General :: How Do Startup Managers Work

I was watching a video on "best apps of 2014" on YouTube, and a startup manager was one of them.

I downloaded it and tried it out with Skype, rebooted, Skype wasn't under the running apps, barely anything was actually. i thought it worked with the skype, until i rebooted the phone again with nothing selected to not start at startup, and all the same - few - visible processes were running. disable some of the rarely used but always running background processes. for example, a game that i use and enjoy, but not every day and would like it to run upon my request. thats what i thought startup manager would do.