Usage: When Link's hearts are full, swing the sword.
Link's earliest sword technique, the Sword Beam fires a projectile attack. This move only appears in 2D games.
In The Minish Cap, Link learns this attack from the swordsmaster, Grimblade.
Misscellaneous Notes
OoT's Sword Beam
Originally the Master Sword in OoT was also capable of using the Beam Sword technique, although this feature was removed from the final version of the game. Some evidence is found in the picture below.
Beta Version of OoT showing the Sword Beam technique