Shido has the mysterious and unique ability to seal the powers of the Spirits into his body, which was first discovered by Ratatoskr sometime in the past when Kotori was turned into a Spirit. This power was bestowed onto him by the First Spirit, Mio Takamiya. Mio gave Shido the ability to absorb more power in order to gradually bestow her own abilities onto him, with each other Spirit acting as intermediary vessels to facilitate the transfer.
In order to use his ability to seal a Spirit’s power, the Spirit in question must meet certain conditions or requirements. Those conditions or requirements are that the Spirit in question must show affection, trust, closeness, and intimacy towards Shido. Once these conditions of love are high enough between Shido and the Spirit, he can seal their power by kissing them on the lips, creating a path of Reiryoku between them. By the time of Volume 19, Shido has sealed the powers of all ten Spirits, giving him full access to each of their Angels.
Due to the sheer amount of Spirit energy sealed inside of his body, Shido has been shown to possess a level of protection or outright immunity towards the abilities of other Spirits. Theses powers include being able to withstand Kurumi's City of Devouring Time as well as being completely unaffected by Miku's Solo. He was also able to resist Inverse Nia's future describing ability, but needed to actively exert reiryoku.
If the Spirits' emotions become unstable enough, their powers will begin to return to them and leave Shido. This flaw means that he will have to seal them again should their powers fully return to them, but usually this only results in the Spirits temporarily regaining portions of their original power. However, as shown in the cases of Kotori, and much later on Tohka, it is possible for the Spirits to take back their full powers without the need to become emotionally unstable. Kotori seems to be capable of choosing exactly how much of her powers she takes back from him at any given time and so far most of the Spirits have managed to figure out how to form partial Astral Dresses, with varying results.
After the disappearance of Mio's Sephira Crystal, Kotori states that it is unknown if Shido still has the ability to seal the power of the Spirits.