slide ur finger from bottom and mover it to the right if you want the app switcher to pop up faster.
if you have a lot of time and want a simple one swipe gesture, Slide ur fringer from bottom and hold it in the middle of the screen for about a second then your multiple opened apps should pop up for switching or closing
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im assuming you meant lock screen because im sure it works everywhere else [home screen, in apps etc]... since when does multitasking work on a locked screen?