The rules of the Pact state that if a particular Secret is in danger of complete obliteration, all other Secrets must come to their aid. Likewise, if a threat manifests in the world that is strong enough to threaten the Nexus itself, all Secrets must come together despite their differences to protect their common interest.
Every time the Pact is activated, a Pactkeeper must be chosen by election. Once the Pactkeeper has been chosen they are invested with a fragment of power from every Secret, which gives them the authority over every Secret during the time that the Pact is active. This makes them, during that duration of time, the most powerful sorcerer or sorceress of the world.
The cost is steep, however, for once the Pact is revoked and the threat dealt with, the person who was chosen as Pactkeeper dies with it, unable to cope with the sheer amount of arcane power coursing through their system.