Well, I don't know what they are used for in this application: but in general a capacitor can be viewed as a battery. Power goes in, power comes out. Power can be pulled out at a steady rate, even if the power going in is not at a steady rate: much like how the battery in an automobile keeps the electrical power constant even though the alternator is inconsistant.
It does, of course, require that the average power coming in meets or exceeds the constant power out (plus losses to inefficiency)
http://en.wikipedia.org/wiki/Capacitor#Energy_storage_2
I suspect you knew all this and there's something I don't know about this application that makes your question have no answer.