OK here goes!
I don't know the "real" names for what I called 'Quick Energy' and 'Slow Energy' (if there even is one!) but I can tell you about them.
Quick Energy is energy gained from eating foods which contain naturally occuring 'simple' carbohydrate (form of energy) which the body can digest and make use of quickly (hence, Quick Energy!) as well as vitamins and minerals which help to produce an all round, better, more energetic feeling. The most popular and well-known (if you watch Wimbledon) is Banana's; a good source of simple carbohydrates, vitamins B&C and potassium. A banana can be absorbed and used for energy very quickly, which is why tennis players use them to keep their energy up during a match. All fruits contain these kinds of carbs, as they are actually the fruit's natural sugars (yes, for those who didn't know, sugar is a crystallised form of carbohydrate).
Not all simple carbs are good though.
Most un-natural carbs are classified as simple, as they are carbs which have been taken in their natural state, broken down and then re-formed to make something un-natural like chocolate. These "freak" carbs are not good at providing energy in the same way as they would have if they were in their natural state (such as the Banana) but still retain some of their former properties; for example, it holds true for most people that a small amount of chocolate can provide a mental boost, yet for everyone a large amount of chocolate will lead to bloatedness and weigh gain as the body cannot digest and absorb the energy found in the chocolate quickly (hence the "bloated" feeling coming from the elongated form of digestion) and as the energy is released, most other forms of energy will be used in it's place (such as the sandwhich and crisps you ate before the choc bar) as that energy will be better than that found in the chocolate, with the slow crappy energies from the chocolate being laid down as fats for use at a time when there is no other, better energy to make use of.
Complex carbohydrates, such as those found in cereals, are "Slow Energy" forms, as they take much longer to be broken down, absorbed and made ready to use by the body, hence the reason they are good for breakfast as they provide you with large amounts of energy which will be released slowly over time, helping to get you through the morning as well as providing a mental boost which most often, the un-natural "freak" carbs do not give.
So, the effects of quick vs. slow energy in terms of a VLCD.
Well, body fat is extremely difficult for the body to break down, in fact it is pretty much the hardest cell structure to break down. It contains lots of things that can't be used for energy (much of which becomes the Ketones produced by the breakdown of the fats) which have to be expelled. Despite containing a lot of calories, it contains very little usable energy and actually requires energy to do the breakdown process in the first place.
The reason the body will lose muscle while in starvation mode is that muscle is packed with energy, and far easier to break down than fat; hence, the body will 'hang' onto the fat as it is easier, quicker and requires less energy in itself to convert muscle to energy.
So, as you have no glycogen stores (ready-made, ready-to-be-used energy) on a VLCD,3 or 4 times a day for short time you are relying on the little amount of semi-quick energy gained from the carbs after you've eaten a pack, but most of the time you are relying on the slow-to-break-down energy gained from the conversion of the bodyfats.
As such, the body is almost continually breaking down fats to power the body. This slow, steady stream of energy is enough to keep you feeling fine throughout a normal day, but when you do something energetic above 'normal' levels (i.e. go and run for half an hour) then you use up all of the fat-energy your body has broken down, and have to wait while it breaks down more, hence why you can end up feeling extremely tired and run down and generally crap as you literally end up running on empty - that includes mental processes too, don't forget it takes a lot of energy to keep a mentally active mind going!
Hopefully that kinda explains what I mean when I use the terms 'quick' and 'slow' energy, and the difference between how the body can use these various types of energy forms in both good and bad ways