Pedal…
The Fortius has a motor unit that provides the resistance braking, but this also allows it to drive the back wheel. It can therefore actually simulate downhill, although for safety reasons it only does it for about 20 seconds before it pauses, so as not to leave it spinning the back wheel at 30mph and decapitating the cat if you passed out or something.
In reality, it is a trainer, so freewheeling for the descent is a tad pointless, so you do as you would in the real world and pedal as fast as you can to go down hill. The Fortius only really simulates around 20-25mph on a steep downhill without pedalling, so you can easily still push about 200watts on the descent. In that respect it’s harder than the real thing, as you can’t rest on the downhills too much.