Power of waking

The power of waking (眠りし心を解放する力) is an ability introduced in Kingdom Hearts 3D: Dream Drop Distance and plays a key plot element in Kingdom Hearts III, described by Yen Sid as "the power to free a heart from its sleep".

Kingdom Hearts 3D: Dream Drop Distance
Sora and Riku undergo a Mark of Mastery exam that will not only name them Keyblade Masters, but will also grant them the power of waking by unlocking the seven Sleeping Keyholes, thus awakening the Sleeping Worlds. However, Sora loses this power when he is corrupted by darkness and nearly becomes a vessel for Xehanort. Riku uses the power of waking to dive into Sora's heart and defeat the Armored Ventus Nightmare, saving Sora's heart. Then, within Sora's heart, Riku finds himself on the Destiny Islands, where upon listening to three questions and giving his answers, Riku finally awakens Sora.<!--

the purpose of sora and riku's exam is to gain the power of waking, and riku uses it to wake Sora:

Yen Sid: If we are ever to strike down Xehanort, we need the individuals King Mickey spoke of in his letter. We must lead them out of sorrow and slumber, and back to our world. To do so, seven Sleeping Keyholes must be found and unlocked, and a great power retrieved. As you know, every world is walled off from the next, preventing travel between them. In the past, you could bridge these gaps because the walls were broken, or because you could open special Lanes with your Keyblades. But your new goals, the "Sleeping Keyholes," are harder to reach. You'll recall, in your first journey, that you brought many worlds back from the darkness--but some never returned completely. They still sleep, cut off from all outside channels. Not even the Heartless can enter.

Yen Sid: You must understand, this examination is in no way how the Mark of Mastery is usually found. However, in light of what they must do next... it was a necessity. If Sora and Riku complete their test, by finding the seven doors corresponding to the seven pure lights, they will return home with a new power.

Yen Sid: In your Mark of Mastery exam, you were to unlock seven Sleeping Keyholes. By doing so, you would awaken those worlds from their prison of slumber, and also acquire the power to free a heart from its sleep. Riku, you unlocked those Keyholes within Sora's dreams. Therefore, it stands to reason that you now have the power to awaken Sora's heart. Mickey: You want him to dive back into Sora's sleep? But Master, Sora's heart is down in the darkest abyss. If Riku's not careful, he might just get trapped down there with him. No... I'll go instead. --note: "power to free a heart from its sleep" probably "power of waking" in original JP script

Yen Sid: More than anything, I am grateful to have you both back from Xehanort's deception unharmed. And I am grateful to Lea, whose spontaneous actions turned the tides. I am also deeply sorry, for failing to percieve the danger and throwing you headlong into a perilous test. This experience has revealed many hidden truths, and we must gird ourselves for the great clash with darkness that lies before us. I believe we need a new Keyblade Master, one with a new kind of power. Sora and Riku, you both deserve the honor. However, one of you braved the realm of sleep again to unlock the final Keyhole and save a friend. Riku, I name you our new true Keyblade Master.

Ansem the Wise: Yes. A clue, I hope, to finding yourselves or your lost friends in your hour of need. The heart has always been quick to grow. Each exposure to light, to the natural world, to other people, shapes this most malleable part inside of us. Nobodies are not different from us in that manner. Sora was the only one able to return to his human form without destroying his Nobody. That is a statement to the love in his heart for other people, and the bonds that tie them together. Perhaps...he has the power to bring back the hearts and existences of those connected to him-- to recreate people we thought were lost to us forever. Our most precious treasures--even an empty puppet--the trees of the forest, and the petals on the wind--there are hearts around us everywhere we look. And it does not take superhuman powers to see them. Surely we remember as children the way our hearts made everything seem so shiny, and perfect. Sora has a heart like that--uncorrupted, willing to see the good before the bad. When he sees the heart in something, it then becomes real. When a connection seems broken, he may have the power to mend it. He has touched countless hearts, he has accepted them, and he has saved them. And some of those hearts have never left him--whether they fell into darkness or were trapped there--whether they sleep in the darkness of Sora's heart, or were welcomed into its warmth--they can be saved. All Sora needs to do is be himself and follow wherever it is that his heart takes him. It is the best and the only way. The rest is in there.

Ansem the Wise: You see, by defeating the Nightmare imprisoning Sora, you freed him.

Riku: You mean...the Sora wrapped in that black phantom?

Ansem the Wise: Then, you came here, and you were questioned by three young people. That was the final key to awaken him.-->

Kingdom Hearts III
Sora spends his time traveling through the worlds to rebuild his strength in the hopes of reclaiming the power of waking, but it is not until facing off with Vanitas at The Land of Departure that Sora realizes, thanks to Ventus's help, that the power was sleeping inside of him.

After restoring himself in The Final World, Sora uses the power of waking to save his friends from the Lich, using it to travel from world to world and track each of his friends' hearts down. The young Xehanort warns Sora about using the power of waking incorrectly, stating that it is meant to be used for "traversing hearts to reach worlds, not for traversing worlds to reach hearts," and that Sora using it in such a way comes with a high price that he has already paid through said misuse.

As a result of tracking his friends' hearts down, Sora is able to send them back in time to their arrival at the Keyblade Graveyard, erasing their previous failure from existence. Thanks to the arrival of the Lingering Will, history is kept from repeating.

After the final battle with Xehanort, Sora uses the power of waking to look for Kairi's heart to bring her back. However, as Young Xehanort had predicted, Sora meddling with time through his misuse of the power of waking takes a toll, causing Sora to vanish into another world as the price.

Upon arriving in the Final World and explaining his situation to Chirithy, it explains that the power is waking is supposed to be used to wake and restore sleeping hearts, not to restore them after fading from existence. Because Sora did that six times already, he changed history which is a forbidden act and if he saves Kairi, he'll lose his powers and vanish from the world.