The Skyward Strike is a sword technique from The Legend of Zelda: Skyward Sword. Taught to Link by Fi Inside the Statue of the Goddess, this technique unleashes a powerful wave of sacred energy with various effects. In order to deliver a Skyward Strike, the Wii Remote must be held upwards, allowing Link to focus energy in his sword. When the energy build has reached its zenith, it can be unleashed, the wave of energy being at the same angle in which Link swings his sword. When used in conjunction with a thrust attack, the attack is identical to a Sword Beam. If a Spin Attack is performed while a Skyward Strike is charged, the attack's radius will be increased (an effect similar to the Magic Spin Attack) and it will be able to activate objects such as Goddess Cubes, but will not launch a projectile or deal any additional damage.
The Skyward Strike's primary purpose is to activate things such as Goddess Crests and Goddess Cubes. It can also be used to draw symbols on various objects such as Goddess Walls. In battle, it can be used to deliver a ranged attack, allowing Link to stand at a safe distance while defeating enemies that require a strike of a particular direction. Skyward Strikes can be used to easily knock an enemy onto its back, allowing for an opportunity to finish them off with an Ending Blow.
The time required for a charge is lessened significantly when the Goddess Sword is transformed into the Master Sword. During Hero Mode, the time required to charge this attack is initially lessened to what it was at the end of a normal game. After the Master Sword is completed, the charge time is removed altogether, the distance the beam can travel is greatly increased, and the beam itself changes in shape and size.
Both Demise and Ghirahim use variations of the Skyward Strike in battle. Ghirahim's Skyward Strikes bear a orange hue, and like his many other magical abilities release diamond particles as it travels but are otherwise identical to Link's. During the second stage of the battle against Demise, Link is unable to use the Skyward Strike due to the evil energy suffusing the battlefield. However, both Link and Demise can store the energy of thunderbolts in much the same way Link performs a Skyward Strike, allowing them to launch lightning at each other. Also, Tentalus cannot be defeated with a Skyward Strike.
The Skyward Strike appears in both Fi's Goddess Blade moveset, and both Link's Hylian Sword and Master Sword (which is its own unique weapon type separate from Link's Hylian Sword) movesets. As part of Link's Great Fairy moveset, the Great Fairy can unleash Skyward Strike slash waves by borrowing Link's Knight's Sword during one of her combos, however she can fire multiple waves instead of one, while Link humorously dodges out the way of each slash.
Master Sword's Lost Skill
Given that Link never uses the Skyward Strike when wielding the Master Sword in games that take place after Skyward Sword (save for the Sword Beam, however the requirements to perform it are different then in Skyward Sword), it is possibly that the skill was lost or forgotten over time (and could serve as an in-universe explanation for its absence in games created before Skyward Sword). If true it is possible that it may have been rediscovered (should it appear in future Zelda titles created after Skyward Sword) by Link at some point. Another possibility is that when Fi went into her eternal slumber at the end of Skyward Sword, the Master Sword's ability to preform Skyward Strikes was lost and/or weakened to the point it could only unleash Sword Beams and required Link's strength in order to perform it (thus explaining why Link can usually only fire sword beams at full health). The ability itself could have also been lost or reduced as a result of the Master Sword being re-forged (like in A Link to the Past or A Link Between Worlds) over the years.