There are wheel speed sensors for that. Keeps you from going forward crashing into the front and also from stopping and falling off. Watch the video and the reviews explains it along with showing it in action.

Man I saw this back in Oct 2016 (Cool but way too expensive)
https://www.dcrainmaker.com/...tober-23rd-2016.html

If you hop on a treadmill when it is turned off, you can push against the arm pads and spin the belt by trudging forward. There is a lot of resistance in this off state. Then, when you turn on the treadmill and set it to a speed, it starts spinning it to run by itself, in essence, propulsion. So, the treadmill can modulate itself to requiring a lot of force for your bike tire to spin the belt all the way through no resistance, through to propelling you.

It seems like a pretty simple setup.

Your wheel speed has never directly correlated to Zwift speed...only your power output. If the trainer is set to high resistance, 500W may only spin your wheel at 6mph, while at low resistance, you may only need 25W to spin at 6mph. Zwift doesn't care about wheel speed, just wattage.

(if you are using a dumb trainer, it uses wheel speed to calculate power, but assumes that you have proper resistance applied or have a good spindown calibration)

Yes, the key point is 'push against the arm pads'. With zero incline there is almost zero resistance on the bike to prevent it from moving forward while you pedal. If you set up a big fan in front of the treadmill blowing on you, you'd have that headwind air resistance. Or you could tie a bungee cord to your seat post and stretch it to the wall behind you to give resistance.

Of course, but with the treadmill off, there is loads of resistance. I was simply making the point that there are other sources of resistance besides slope.

I do not know if that is how this works in bike mode, but seems to fit with the videos. It seems like you would need some nominal incline or else it would require something like you mentioned (fan or bungee) to keep from going over the front no matter the resistance applied.