While in his transformed state, Rōshi creates a small volcano that explodes violently, sending molten rock flying up in all directions like a giant flower. The heat of the lava is enough to burn tailed beasts as strong as Gyūki.
The summit of the Mountain of Flowers and Fruits (花果山, Ja: Kakazan, Ch: Huāguǒshān) is the birthplace of Sūn Wùkōng, the main character of the Chinese novel Journey to the West. The mountain is located on a volcanic island. The Four-Tails, of course, is based on Sūn Wùkōng and the Dragon Ball series, which in turn is also based on Journey to the West.
When written as 火山, kazan means 'volcano'.
In an anime exclusive sequence, Rōshi, while offscreen, used what appears to be this technique when colliding with Kisame's Water Release technique.