It's a simple application of Ohm's law. Watts = (Voltage x Voltage)/Resistance. So an amplifier with 40V rms output will deliver 200 watts into 8 ohms, 400 watts into 4 ohms and 800 watts into 2 ohms, all things being equal (which they usually aren't). Generally speaking, you won't find 2 ohm speakers. This rating is meant for two 4 ohm speakers in parallel.

Bryan