its not voltage that changes, it will remain 12 (voltage fluctuates with rpm).
Think of the voltage as a 5 gallon bucket, inside the bucket you have watts (55) for the sake of simplicty here we will say watts and amps are the same thing..they are not watts/volts=amps so a blaster can push 55 watts at 12 volts = aprox 4.5 amps..anyways before I confuse you..
the 5 gallon bucket is the voltage, inside it you have watts or amps, the headlight takes so many watts out of the bucket, the taillight takes so many watts out of the bucket.. you can only take 55 watts out of the bucket and then it's empty. if you try and take more watts out than the bucket holds..like running 80 watts of lights, the bucket can only give 55, but the bucket realizes you are trying to take extra, this upsets the bucket and the bucket gets hot. if you get the bucket to hot to many times it melts, ten it has a hole, and cant provide any watts and you have to buy a new stator because you melted the old one