Had the same questions before I got mine.
The bridgeboard actually boots along side with the Amiga, in parallel, but you never see that. This is done by the janus.library which is in the Expansions drawer. When the ss hits Binddrivers, the bridgeboard starts to boot. You still see the amiga booting as normal.
Then, when on WB, you just click on the "PC" icon (from the janus distribution) which opens a screen to the bridgeboard. Both machines work at the same time, and you can switch between pc and amiga screens on the fly.