From the Super Mario Wiki, the Mario encyclopedia
Balloon Yoshi is the first transformation found in the game Yoshi Topsy-Turvy. In this form, Yoshi becomes a hot-air balloon and slowly floats downwards. By tilting the Game Boy Advance left or right, Yoshi will drift to that corresponding direction. Coins, spikes and other obstacles are usually found on the way down. It is similar to the Hot-Air Balloon Yoshi transformation in Yoshi's New Island.