How does Connection Pro work with a customer provided router?

We have static IPs and our Comcast provided SMC modem/router in "passthrough" mode so we can use our own router and we're considering using Connection Pro to provide backup.  I know the static IP assignments won't follow the switchover to the Cradlepoint but how exactly does the switchover occur in this situation?   Actually, I'm still not understanding how the switchvover would work even if we used the SMC as the router.  Does the cradlepoint tell the router to change the default route to itself?  Exactly how does the Cradlepoint detect the outage in the first place?