an Ohm is a measure of resistance. this may shed some light on the relationship between the amplifiers output and resistance of the speakers.
http://www.outrageousaudio.com/page_files/wiring_subs.pdf
In a simplistic explanation "Amplifiers provide the electrical pressure in a circuit; ohms measure the resistance, or load, against that pressure; and watts measure how much power is released as work. So, using one of those math formulas, an amplifier that provides 100 watts of power through a 4-ohm speaker, will produce 200 watts through a 2-ohm speaker, because it's easier to push that reduced load."