You can see it in action as the primary strategy to beat this beetle boss. To activate the Whirlwind, you have to tap that icon in the upper right hand corner of the screen to bring up the yellow targeting line. Then you blow into the Nintendo DS microphone to set off a gust of wind in the direction of the yellow line.
I guess there is something potentially spoiler-ish at the end there when that green shard shows up. I have no idea what it is, but the more research-savvy among you might. In which case, you're probably already riddled with spoilers.
The Legend of Zelda - Spirit Tracks: Forest Temple Boss Fight [Zelda Informer]