When a router or switch is booting, the routing protocol process (rpd) is not running
Effectively this means, that while RPD is not running, the backup route will be in use. This would be true during boot up of the router and also when the routing engine is running as the backup routing engine. Once the RPD process is started however, this configured backup route is no longer valid.
Thanks for your answer, Anton. I guess I should've been more specific about my question, I am interested in finding out if the router is able to perform packet forwarding between hosts that are using it as the default gateway and the rest of the network, using the backup router, before its own routing process starts, or when for whatever reason the routing process fails to start.
I would say no. This command is used to give the Routing Engine some routing while RPD is down. The route will point of the management interface (typically FXP0 or similar interface) which is on the routing-engine itself. You can also not route traffic between this managment interface and the physical interfaces in the chassis.