For me it was completely involuntary and without my awareness until well into therapy. I wasn't really even consciously aware of 'lost time', or I didn't term it as such or something. It was simply my normal or my experience of life to not really have any control about what happened, what I did or where I ended up. Inconsistent experiencing was 'life'.
In therapy, after getting to know and sharing information and experience with the different parts of myself, I was able to be more conscious of the switching process. I could give permission and 'step aside' for another alt to come forward. (On the other hand I couldn't prevent it if it was going to happen).
So I guess I wouldn't say switching ever became voluntary... but it definitely became conscious.