It is based on Box2D 2.2.1 and adds more features:
Joint release/grab, WeldJoint, WheelJoint, FrictionJoint, Edge shape support, more properties and methods (gravityScale, getMass(), getInertia(), angularVelocity, etc) and Body fixture iteration.
Here are some videos showing usage of PulleyJoint and RopeJoin:
Have fun! (oh, Qt 4.7 or 4.8 only!)