Please see what the manual says: http://www.cameramanuals.org/pentax_...potmatic_f.pdf
On page 1 there is a description on how the meter switches on by removal of the lens cap. Also on pages 20 and 21 there is a description of the "stop-down lever" as Pentax calls it.
If the needle is up, regardless of the state of the stop-down lever, the meter is active.