I think you can set it up as a toggle (press once for low DPI, press again to go back to high). I don't actually use it.
I think for the RAT you just hold down that red button for low DPI and release it to go back to normal which is cooler, IMO. I was trying to figure out if it was possible to write a macro that would mimic the function of the RAT's sniper button but I don't think it's possible.
Edit: I just read that this has been a feature in MS Intellimouse Explorer for years. Maybe you can try it if you have a MS mouse?