Mamiya 645 PRO TL - Manual (Page 7)

10
.
Other Features
After focusing, the mirror can be locked up before
making an exposure. This is convenient when the
camera is used at slow shutter speeds and is mounted
on a tripod for telephotography, copywork, etc.,
since even a very small amount of vibration should
be eliminated.
Shutter will be released 10 seconds after shutter
button is pressed.
When making time exposures, the battery circuit is
automatically disconnected, to save battery power.
This is especially useful for astrophotography.