Usage: Press and hold the sword button. After charging, release.
This is one of Link's common attacks since A Link to the Past when his uncle taught him the move.
In The Minish Cap, Link learns this attack from the swordsmaster, Swiftblade. The swordsmaster, Scarblade, teaches the Spin Attack Charge Upgrade, allowing Link's Spin Attack to charge more quickly.
Also in TMC, charging the sword fills Link's Spirit Gauge. The swordsmaster, Splitblade, teaches the Spirit Gauge Charge Upgrade, allowing Link's Spirit Gauge to fill faster.