While diving, keep the lower button pressed to add a bookmark (timestamp) to the active log for later reference.
Bookmarks save the following info: time stamp, depth, temperature, pressure if tank pod is used. Data can be seen in DM5 after the dive.
If you keep the lower button pressed while the compass view is active, both a timestamp and the current compass heading are stored to the active log.
In compass view pressing down button long will lock the bearing.