Gazelle taillight Power Vision user manual
The Gazelle Power Vision taillight has no independent manual.  As it is only sold on a bike, the manual for the bike contains the information you need. 


You can determine if there is an automatic function built in to the light by checking the power switch. If the power switch only has 2 positions, you have a manual only light. If there are 3 positions, then the light has a sensor function.  The function you are thinking of as a lock, may be the auto function. If not, you appear to have the manual version. 
If the sensor function is active, then the light will turn on when the bike is moving. It will remain on 1 minute after motion stops. Some models also have a light sensor, and will stay on longer in the dark. 
The battery indicator is automatic. It will begin to flash when the light calculates 10 hours of battery life are remaining.