[wayner]

Following on what drkshdw said,

If you want to do a fasting workout, especially an endurance workout, you've got to keep your exertion low enough to not burn up glycogen more quickly than you body can convert fat.

If you are successful in doing higher intensity fasting workouts, my understanding is that it is because
a) You are already in really good shape and your system has become more efficient in converting
b) your intensity is staying low enough
c) your actually starting to burn protein form your muscles and decimate them

In either case, in simple terms what I have been told is that the body uses energy in this order (and can switch in flight or fight mode but Ill leave that for another discussion

The conveyor belt:
1) burn glycogen stores in the muscles (a type of sugar)
2) when depleted call on glycogen stores in the liver to transfer to the muscles
3) when depleted call on fat to be converted to glycogen
The conveyer belt from fat to liver to muscles.

After your workout your body continues to feed the muscles and fill them back up, so after the workout what you eat can continue to fill up the muscles needed sugar stores or let the fat do it depending on intensity and what is needed back.

Note:
If you have a high sugar level in your blood and are doing high enough intensity, it will use that source directly instead. (Again leaving acrobatic/anarobic discussion for someone else to cover).

Also an easy but lengthy walk may be able to be fueled from fat alone since the rate of use is slow enough for the conveyer belt to supply it.

CAUTION:
If you work out even more intensely or for too long you may fall way behind and when you get home you'll uncontrollably eat absolutely everything in the fridge and then sleep for three hours (and gain a lot of weight)

Also, using glycogen from the muscles in normal. Burning protein from the muscles is not.

Being overly tired DURING your workout is a good sign that you need to eat.
Headaches and lightheadedness, (or passing out) means you've been stupid about not eating)