As we noted, the predictive back gesture shows an Android user a preview of where he will be taken to if he uses the back gesture. There are only three possible destinations: a previous page in the same app, a previous app, or the home screen. The back gesture on Android requires the user to slowly swipe left from the right side of the screen until a left-facing arrow inside a circle appears. Once the user pulls his finger off of the display, he goes back to the previous page. The same thing is accomplished by slowly swiping to the right from the left side of the display, letting go when a right-facing arrow inside a circle shows up on the display.
The Predictive Back gesture for Gmail and Google Calendar