[ODE] Adding a torque about a hinge

[Lukasz Wiklendt]

I had a look at dJointAddHingeTorque and found that it simply adds a torque
in the direction of the hinge axis to one body and the reverse to the other
body of the hinge.

Could anyone please help me understand how this works? When the torque is
applied to each body, is it applied about their center of gravity? If so,
how does this translate to a torque about the hinge?

Thanks,
Lukasz.