This was done over a period of 3 to 4 months. I’m not sure how well the body likes gaining muscles mass that fast, but he hardly gained any fat in the process. Which is really unsual since when your normally do bulk in bodybuilding you gain quite a bit of fat too. This probably would help hard gainers, or people who have trouble gaining weight, like muscle mass and tend to be thin. He is also using only body weight exercises although I think that includes kettlebells which for some reason is often included in bodyweight exercises, although its a free weight. Hopefully someone can answer why that is.
He explains it more on his video and blog here,