Muggles' Guide to Harry Potter/Magic/Tongue-Tying Curse

Overview
The Tongue-Tying Curse causes the subject's tongue to roll up in his mouth, preventing speech.

Extended Description
While it is similar to the Langlock jinx, the Tongue-Tying Curse has a different physical effect: Langlock causes the subject's tongue to stick to the roof of his mouth, rather than curl upon itself.

There is suggestion in the books that the Tongue-tying curse may have a more permanent effect, preventing the subject from speaking of a particular topic, but this is never confirmed.

Analysis
The Tongue-Tying Curse is left on the entrance to Number 12, Grimmauld Place by Mad-Eye Moody, when the death of Dumbledore forces them to abandon the use of that house as Headquarters. Theoretically, Snape could at that point reveal the presence of Headquarters to anyone he chose, including Death Eaters, as Dumbledore's death has left him a Secret-Keeper. We are told by other members of the Order of the Phoenix, however, that Moody left spells that would prevent Snape doing that. As the curse's action on Harry when he enters the house is quite brief, it could not prevent Harry from speaking of Grimmauld Place; the only way it could deter Snape would be if there was a long-term effect that was not immediately apparent.

Greater Picture
While it is never explained exactly how long the Tongue-Tying curse's effect may last, the author may have introduced it as a means to deflect readers' probing questions about Snape's loyalty, which remains unknown until the end of Harry Potter and the Deathly Hallows. If Snape had been a traitor, then it would seem logical and probable that he would have revealed how to enter Grimmauld Place to Voldemort and the other Death Eaters. Following the demonstration of the Tongue-tying curse's effects on Harry, readers might assume that this was the reason for Snape's silence.