Alt-tab doesn't really exist in Android: there is always the extra step of opening the recent tabs window. Pintasking solves this by letting you pin apps to the screen as small buttons that can be pressed at any time from any app.

When you press the pin, the app opens, and the pin turns into a pair of arrows that can be pressed to quickly return to the previous app or window. It's very handy if you need to frequently switch between two or more apps.

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