As the yo-yo reaches the end of the string it will be swinging towards your body.
Extend your freehand index finger and allow the string, slightly above the yo-yo, to hit your freehand index finger.
This will cause the yo-yo to travel back up and over your finger.
The string will wrap around your finger and the yo-yo will swing into the string hanging down from your throwhand.
This is the bottom mount. To get out of it leave your freehand stationary and pull your throwhand up, this will cause the yo-yo to reverse the motion is just traveled, going back up, swinging off your finger and leaving you with a sleeper.