The following information is based on events in The Secret Society of Seven Sorcerers and contain spoilers.
The unlocking spell (Expositus) is a spell that, when cast on a locked door, lid or simply a lock, unlocks it and may be able to open it as well, exposing the locked space's contents.
Examples of usage
The word expositus is Latin and means "exposed" or "expose", which is technically a synonym for "reveal." The variant spell expositus modo takes applies the word modo, "instantly", to the spell, presumably wielding the effect of any door opening fully instantly when the spell is cast on it.