When I say "you do not have to lean at all to turn", I was not clear. What I mean is that you can look like the left photo, rather than the right photo: http://i.imgur.com/K4vTdDm.gif

I read your comment to imply that the rider's body lean is what initiates a turn, rather than their handlebar input. Or that the rider must actively lean (their body), rather than allowing the bike to lean them over.

Your other reply to jsprogrammer says that "it's pretty obvious that the passenger needs to be shifting its weight to achieve that." But that's precisely what I'm saying is wrong. No weight shift is required at all to do any street-riding turns. When the handlebars are steered, the bike will force you over. When a seated driver of the C1 turns the bars, the seat will move their body over, they don't have to do any work.

If you're on a racetrack, then yeah, shift your weight and eek out sharper turns, but you'll look silly shifting your weight around when riding on public streets or a highway.

Cheesy video that shows it better than I can explain. Skip to 2:25. https://youtu.be/8_5Z3jyO2pA