IIRC, with Topo GB and a Route loaded through Mapsource, to start the ride, I had to load the route through the ‘Saved rides’ menu and press the ‘Start’ button. Along the Route, the unit would bleep some distance prior to a direction instruction, and then again immediately before the turn.
If I went off-course, recalculation would happen and the unit would bleep to inform me the next instruction was a ‘U’ turn.