The Palm official page on resets
here.
A hard reset is simply, holding down the Power button (in the case of the Treo 650, it is the red phone button at the right). Keep holding down the Power button while you poke the reset hole in the back of the Treo near the battery. Keep holding down that Power button until you see the gray Palm logo screen (not the PalmOne logo start up screen).
But, in my view, hard reset doesn't always wipe everything out if you are experiencing problems.
A Zero out reset, will be the true hard reset which will wipe everything out back to the fresh out of the box settings. It is much more tricky to do.
Hold and press down on the following buttons: Power (red phone button), hotsync cradle button (make sure it's connected to the Treo), up button. All the while use the stylus to poke the reset hole in the back. The trick is to hold the stylus with your mouth.
If you do it incorrectly, you will get the rainbow bootloader screen. Simply do a soft reset and try again.
I have provided a video of what it looks like when performed correctly in
my Palm Discovery blog posting.