The theory behind spikes is rooted in the physics of motion - as penned by Sir Isaac Newton. As suggested, they help "focus" the energy of the movement of the speaker - which is actually a motor in principle.
The spike offers a poor impedance match to acoustical vibrations, which will not readily pass through. I have heard of, and seen this concept taken to the extreme - people actualy using them under amps, preamps, etc. While there can be some benefit to having an isolated source (CD or turntable), I have not experienced anything earthshattering using them.
While the effects are subjective, it is a very easy test to perform. I have heard some differences, and other times none at all.
