you want to eat carbs for two reasons. One is to kick you out of ketosis so you aren't using ketones for energy. This will force you to burn glucose and deplete glycogen from your muscles even further. Also lyle stated in his book that you should eat some form of carbs five hours before the workout to get the enzymes required to process carbs working. Apparently that's how long it takes to change back over, much the same way you have to switch over to ketosis after being in a glucose burning state.