In the Legend of Zelda: Twilight Princess, occassionally my sword (or my tail when I'm a wolf) will flash, and an accompanying sound will be played on my Wii Remote. What does that indicate?
If you're referring to what I think you are, you might notice that this sound always follows a spin attack.
Essentially, the spin attack (and tail attack in wolf form) have a small cooldown period between uses. Your sword / tail flashing means you're ready to perform a second spin attack.