The primary advantage of an FRC dive from an energy standpoint is that you can sink the whole way down. However, from a buoyancy standpoint, you can only sink the whole way down if you are not using a wetsuit.
Further, to ascend from depth during an FRC dive requires extreme vasoconstriction. Otherwise, the legs burn too much O2. Since the O2 supply is so small, you can't afford to burn O2 with the legs. The legs must rely on Creatine & ATP & Myoglobin as an energy source.
To maximize vasoconstriction, the skin on your legs must be in direct contact with cool water. Another reason why no-wetsuit is a must.
Of course, it is possible to do an FRC dive with a wetsuit. You just can't go very deep.
My 5mm elios yamamoto suit has the buoyancy of an 8mm picasso. To do FRC dives in it, and sink the whole way down, requires 19lbs of weight (8.6kg) for my body type. I once did 38m FRC in that config and it was EXTREMELY difficult. Imagine how heavy I was at the bottom, and with empty lungs.
There are three wonderful advantages of FRC dives:
1. Because you sink the whole way down, there is no debate or argument about descent technique. In constant ballast, it is MUCH harder to perform a correct monofin stroke on the descent, while upside-down. During FRC dives, no kicking is needed on the descent, which greatly simplifies the problem of what technique to use on the descent. On the other hand, ascending with a mono is easy, so that is the same as before. Also, because the FRC descent is so easy, it is much more enjoyable than on inhale dives.
2. There is no N2/CO2 narcosis or O2 toxicity with FRC dives. This means there is no woozy/drunk feeling, and there is no 'feeling of impending doom' which occurs from oxygen toxicity.
3. Because your chest is very squeezed at the bottom, you feel like a seal.
Of course, there are drawbacks, which are primarily:
1. Equalizing is much more complicated
2. Chest must be very flexible
3. Basal metabolism must be lower (since O2 store is smaller)
4. No wetsuit can be used
5. In general, the depth you can dive to is a bit reduced (although in theory with proper training this can be overcome)
Eric Fattah
BC, Canada