Calling OBS as plate umpire

[DaveASA/FED]

Ok, OBS delayed dead ball, left arm straight out held in a fist. As PU I pull the mask off in my left hand as soon as something happens following a pitch. So long hit to outfield R1 from 2nd is rounding 3rd, I have mask in left hand (along with indicator) I see OBS with 3rd baseperson and signal OBS, I notice as runner crosses the plate I have onknowingly switched my mask to my right hand as I hold the left out in a fist. never noticed it before and don't think I did it on purpose!

So what do you all do in this case. Keep the mask in left hand and just hold mask straight out? Or do you move it and hold the fist out there? Just wondering what the actual proper mechanic is for this.

Thanks