Well it not a button. Did anyone ever notice the spinning wheel at the bottom of the optical mouse. Wonder what its for. Its around the rim of the optical light very hidden.
That little wheel around the optical sensor adjusts the tension of the clicker. Set the wheel to "+" and there is a bit more resistance to clicks; set it to "-" and clicking requires a bit less pressure.
sweet... i always thought it was the sensitivity of the movement, but that makes more sense... i use an intellimouse (yay for 5 buttons) so i have no use for the pro mouse... i like to look at it, though...
Speaking of instructions, ever seen the difference in instructions for setting up a PC versus iMac? PC comes with a book the size of LotR while the Mac has 2 sheets of pictures(and this includes where you should hold)