Firstly, he creates a clone — which resembles a chibi version of himself — made from the oil and water in his body. The water in the clone is surrounded by a thin layer of oil and due to this structure, the clone's temperature can easily be altered from cold to hot and vice-versa. When it exerts itself, the outer layer of oil heats up and evaporates the water inside. As the internal pressure builds, the clone expands until it violently explodes and the water vapour is released. This vapour then cools down as it rises and as a result, turns into hail that cools the clone, causing it to shrink to its original size and start the entire process again.
Despite its constantly increasing size, the clone itself remains very fast throughout the technique, enough to elude even Gaara's chakra-infused sand. This high-speed movement also allows it to heat up very quickly and to more easily inflict damage using an axe-like liquid blade that it is able to form on its arm. However, using this powerful technique leaves the Mizukage in a severely weakened and vulnerable state.
The name of this technique and its use of a "youthful" clone of the Second might be a reference to the famous Japanese animation film Steamboy as the 'bōi' part is probably meant to imply the English word "boy" and "jōki" simply means "steam" when written as "蒸気".
The continuous cycle of this attack is very similar to that of the water cycle.