It works basically the same way as other Metromile policies, meaning you pay insurance based on the miles you drive. The difference is that you don't pay for the miles the car accumulates while it's being driven by someone else through Turo. Turo's insurance policy covers vehicles while they are being used by Turo users, so this helps hosts avoid paying for redundant coverage, presumably saving money.
Metromile tracks mileage using its Metromile Pulse device, a meter that plugs into your car's OBD-II port. Cars built before 1996 can use an adapter that plugs into a cigarette lighter or 12-volt outlet. Metromile does collect data using the device (a concern with any third-party device that plugs into your car), though it claims the data is secure and won't be sold to anyone. That might be a bridge too far for some users, though it does allow Turo hosts to check things like fuel economy and engine codes.
The service is rolling out first in Illinois, soon to be followed by California, Washington, Oregon, Virginia, New Jersey, Arizona and Pennsylvania. Turo says 40% of its hosts live in those eight states.