Mamiya Rb67 mirror up question.

I was reading the manual for the RB 67 Pro S for mirror up operation. It
says that you first put the mirror up and then trip the shutter with a
shutter release cable, or if you don't have a release cable you just move
the mirror up button back and it will trip the shutter. Now my question is,
Would that not defeat the purpose of the mirror up operation? I mean how
can you trip the shutter by grabbing a little knob that is hard enough to
turn when it is not during an exposure, and maintaining the camera still
enough to trip it? I have a cable release but I'm asking about that time
when you forgot to bring it along or that doesn't work for some reason.
Have I got this right? Thanks. Ric.
P.S. I'm assuming the obvious that the camera and shutter are wound and all
the dark slide and common sense things are taken care of at this point.