The lower the Ohm rating (impedence or resistence), whatever you want to call it, the more power that can theoretically be harnessed.
If you wire a D1 at 2 ohms, it won't recieve the same power as if it was wired 1/2 ohm.
Think about it being like the nozzles people put on the end of water hoses. Towards the higher end of the spectrum (number wise) you get the more misty, lower power spray. Scale down to the latter, and you're on the jet setting, or power washing.
Or a water faucet;
Water: Voltage
Nozzle: Resistence
Pressure: Amperage
For the most part, no matter what you set that nozzle to the same amount of water (energy) is going to get through, though your chosen setting is what initially determines how efficiently that water (energy, again) is going to get where it's going.