I think there are two kinds of spiral bits. One cuts so that it pushes the wood chips towards the router motor. The other pushes the chips away from the router motor. So when do you use one, and when do you use the other? If you have a table-mounted router and a bit that pulls the shavings towards the motor, won't that clog thinks up?

Drick

If you use the Search feature (on the bar that starts with User CP on the far left) you can come up with more comments on spiral bits. One of these can be found at:
http://www.routerforums.com/showthread.php?t=199

After reading these things should be more clear for you, if not post again and someone will provide more information.

Ed