By doing a very quick tap on the shutter button will lock up the mirror but not release the shutter.
It's a bit tricky and it must be a very quick tap.
Next time you press the shutter button the shutter will work as usual and the mirror goes down for the next shot.
This is the same way you can do on the older Pentax Spotmatic.
I would guess that these have very similar or if not the same shutter mechanics.
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