Well, I speculated on that in this post, which posits that it has to do with certain triggers!  One thing that I’d like to mention in addition is that Pokémon seem to be especially responsive to objects that seem otherwise uninteresting.  This is not just in evolution but in all aspects - a simple twisted spoon increases the power of Psychic attacks, for instance.  One thing I wish to test in the future is whether there are some intrinsic properties of these items that compel the enhancements, or whether it is the Pokémon itself responding to the symbolic representation of these items!  For instance, perhaps just the exposure to the stimulus induces the enhancement, so even a picture of a twisted spoon could enhance those Psychic attacks!

Testing such a thing would be quite difficult, but I’m in the process of coming up with an apparatus which might just do the trick!  The idea is that there would be a sensor which could measure the energy input from an attack and provide an output which would give a rough numerical measure of that energy, as well as the type of energy that hit it.  It’s a good conceptual idea, and I’ve been talking with a couple technical engineers about its construction.  The main problem we have right now is figuring out how to build a sensor which could register the energy output without being destroyed!  We’d need something that was basically indestructible but also is responsive enough to register an impulse which could be transduced into an electrical signal.  I’m working on research into that, but I haven’t come across anything yet - hopefully that’ll change!