To save a session, you have to:

1. Starting from the "TIME" screen, press the MODE button (bottom left)
   
   
2. Once in ALTI mode (altitude is displayed), press the PLAY/STOP button (top right)
   
   
3. The stopwatch will start
   
   
4. Once your session is over, hold the PLAY/STOP button (top right) for 3 seconds).

Your session is automatically saved.

You can view your session from the "DATA" mode which is accessed by pressing the mode button (bottom left).