Pentax P5 Manual page 51

3. Recompose your picture and shoot; the subject will correctly be
exposed. The memory lock will automatically be cancelled as
soon as you release the shutter.
Cautions
• When the Pentax dedicated auto flash is used on this camera, the
memory lock does not work.
• If you accidentally depress the memory-lock button with the
camera in the Metered Manual mode, the shutter speed set will
meaninglessly lock and flicker in the viewfinder.