The closer this value gets to zero, the more diffused and dimmer the caustic gets, while the nearer it becomes to 1, the more tightly focused and pronounced the caustic gets. At 1, we have the caustic of a thick, highly refractive piece of lead crystal, while at 0.1 it is more like a hollow glass sphere. We try this by re-rendering the above scene, with a range of values from 0.1 to 1.0 and watching the different caustics we get. Out of range values work also. Numbers higher than 1 just lead to more and more tightly focused caustics.
Negative numbers are just plain weird, but interesting.
Essentially, the object becomes illuminated in all sorts of bizzare ways and the shadow becomes like a photographic negative of itself. Kind of like a 1950's sci-fi raygun effect. It looks strange, and not at all photo-realistic, but if we like the surreal we may want to try it at least once and file away the effect in our mind in case we ever want it.