they follow fire wait, they will fire a beam every time weather you have finished the previus shot or not, this enables you to make beams capable of a overload typ thing, fighter beams also have a diferent implementation of cycle, you need to give the beam shots (its a value in the beam info, look at a post I made recently on TBP forums for an example)
I had the beam draining power the entier time they were doing anything though I supose they shouldn't be draining during warm down, that probly explains the reason why AI don't seem to be useing them as much as they used to.