Halogen bulb is basically a filament or incandescent bulb consisting of a tungsten and gas filled (usually inert gas). By apply a DC or AC current, the filament will heat up and glow. By varying the voltage, the temperature of filament will changed and hence will becomes "brighter". The inert gas will prevent the filament from burning itself out. Thomas Edison was the first person to make a practicle bulb.
A 55W bulb will have a fixed resistance. In order to increase the current flow, the voltage have to be increased. This will increased the temperature of the filament and increase the brightness which by the way is measure in Kelvin.
All the above works very well in theory but in practice, the lifespan of the bulb will decrease drastically when the operating voltage deviate from the original specification.
There are some people who increase the brightness by changing the wattage of the bulb. Normal wattage for the UDM is 55W. I have seen some guys installing 110 W bulb without upgrading the bulb connector, wire harness, fuses and power relay. They usually end up with melted headlight, burnt cable, blacken holder, battery draining and loss of car power.