Second of all, the idea behind this is so simple, and yet really interesting.
So the idea of this machine is based around idea 9.6, which states that "The retracting force supplied by a stretched rubber band is entropic in origin." On the basis of this idea, we can see that it's not the elastic energy which will decrease as the rubber band retracts, but the free energy F.
When we heat up rubber, it causes the rubber to shrink, due to its negative coefficient of thermal expansion. By setting up a heat contraption like in figure 9.3, we can see that by shrinking the exposed side, we cause contraction in the rubber bands exposed, causing an unbalancing in the wheel, resulting in rotation to expose the cool bands to the heat, which eventually leads to a cyclic process, thereby creating out heat engine.
Limitations? I'm sure the rubber bands will eventually snap on us