The frame (or chassis) is in two parts, one tilts and the other does not. The (longitudinal) tilting axis that joins the two is set at an angle to the road surface so that tilting one sub-frame causes the other to steer. The angle determines the tilt-steer ratio. Note that with these designs there is generally only one wheel on the tilting sub-frame. Note also that in the 2F configuration this steering mechanism has something in common with the ox wagon: The two wheels are mounted on a rigid beam axle, and the entire assembly is made to steer.
There are many variations on this basic mechanism. Most are seen in human-powered trikes. Below is an example of a Dutch 1F1T trike that is really the same as the Varna but reversed.
This tilt-steer mechanism using an inclined axis has been developed most ingeniously in the Maurer trike shown below, which is a challenge to anybody's imagination!
As pointed out above, any fixed ratio tilt-steering mechanism has inherent drawbacks, and for that reason my own work in this area has stopped and I do not consider it worth pursuing. The interested reader should consult HPV-related websites for more information.