Yep, like John says the knob just tells the camera to M.UP on the first press of the shutter and capture on the second press. On the next shot you have to press the shutter twice - once to mirror up, second to shoot.
It's not like the Hasselblad H series mirror up, where you can leave it up all day & keep shooting. Yes, it's not ideal.
Good luck with Precision - in my 2 dealings with them it has not been good. No updates, no option to do some preventative maintenance. First trip was for a bent blade on the shutter curtain, second was a major overhaul. Both for the same price though.