A much easier way is:
Swipe all apps closed.
Open and close the settings app a few times (I think two times is all you need).
Reboot.
When phone is back on only settings will be displayed.
As far as it being a screenshot or history of previously used apps. I don't think that's the right way to explain it. If that were true then swiping them closed would do nothing except remove them from history.
In actuality the apps are suspended until called on. Where this is noteworthy is when an app is acting up the only way to resolve some issues beside deleting and reinstalling the app is to swipe it up. That clears all suspended references to the app and allows it to start fresh.
I personally will do this every once in a while just to make sure no app is taking resources that it shouldn't be. An example of this is I used to swipe up the Facebook app as it was a known battery drain. I eventually got sick of doing this and switched to mobile web version and saved the page to springboard.
it is a screenshot and an history AFTER A REBOOT, not while using.
It is designed that way.
DO you want a demonstration ? open Safari in portrait, open a webpage, put it in background, open another app in landscape and then recall multitasking switcher, still in landscape : Safari will be there, but the page displayed isn't the one you just closed but THE LAST to be viewed (and thus screenshoted in landscape).
The same happen when you reboot the phone: a list of recently used app with the last "recorded" image showed.