Originally Posted by lightbulbjim

Assuming it's somewhat like the existing units (I have a Super GPS for comparison) then the phone app will generate the directions (basically a TCX file) and then transfer the file to the head unit via bluetooth. When you get a turn prompt the head unit will beep and display a prompt like "turn right onto Foo St in 80 metres". If you miss a turn then the head unit will automatically ask the phone for rerouting directions and you continue on your way.

Historically the phone app has required Internet connectivity to fetch the directions. In the last week or two the app has been updated to support the new units, and now you can download map regions and generate routes within those regions while offline. This works even for my Super GPS.

My understanding is that what the new units bring to the table is that they also display a map image on the screen as you ride. In the app there's an option to send your downloaded map to your GPS if you have one of the new units. Presumably this is then used as the underlay image.

There's no option to download third-party maps that I can find. My Super GPS is accessible as a USB mass storage device, so if I had one of the new ones I'd certainly try copying some official maps over and then plug it in and inspect the files, assuming they're visible. It might be possible to reverse engineer support for third-party maps.

If you're interested then you can download the phone app and play around with it. You don't need a head unit connected to download maps, generate routes etc.