I am already "super-skinny". I'm doing this for health reasons.
It just so happens that I have the ectomorph skinny-man body type. My bones are tiny: my pinky and thumb meet with slight overlap when doing the wrist size test. That's damn small

. My muscle fibers are short - there's a space between my bicep and elbow when I flex, for instance. My ankles are quite visible, quite unlike some others whose calves magically turn into foot without need of ankle. Evolution seems to have endowed my family line with the ability to walk super long distances in the desert on sugary tropical fruit, as opposed to making us short and tending towards plumpness like many islanders are, or tending towards being short and thick like eskimos.
I used to weigh 120 and am 5' 7" male and now weigh about 114. Nevertheless I can do about 60 push-ups and over 20-pull ups, and that number grows every week. I'm very strong. I also have great stamina. In fact, currently my leg muscles run out before my cardiovascular output, which never happened before. That signals to me to eat more sugars to replenish that loss, but the loss of mucus from my respiratory system was very effective in making me more efficient.
There is actually a method, that I have not tried and can not speak of experience from, where fasting actually makes the body's metabolism more efficient (slower yet doing the same things), and then eating normally will have you put on weight in no time. It is cyclical over weeks and months.
Matt Monarch related a story in one of his blog posts where a man water fasted for 14 days, and after that ate normally and gained "muscle like it was nothing". Link:
Raw Success: Ear Candling (hold your stomachs!!!), Gaining Weight Through Fasting???!, Natural Calm, & Contradictory Views on the Raw Diet... Grrrr...
Now realize Matt Monarch is really skinny. He probably doesn't care. I, too, care more about functional strength, but once I get this way of living on lockdown I'll probably put a concerted effort towards putting on muscle mass. I eventually want to be able to do incredible feats, like 1-finger pushups, climbing massive trees and mountains, holding weird poses for long periods of time, etc. I also want to be able to lift very heavy objects easily. That's functional strength.
I also want the ability to stay up for days in a row without a single drop in energy, as David Wolfe has proclaimed the ability to do. I take that with a grain of salt, but this way of eating makes sense for a number of other reasons, and if I follow this path further I may see special "benefits" like these appear.
BTW - I've read from pretty much everyone that you have to get the toxins out and let weight fall slightly below normal before putting weight back on.
The weight most people have is not real weight anyway. It's mostly water weight, and that is lost almost immediately when going raw.