What does Mat Cauthon's fox head medallion do?

The foxhead medallion is a ter'angreal that vitiates the One Power being used upon it and its bearer. The ter'angreal is not a singular one though, Cadsuane has a similar hair ornament that performs the same function. The medallion does NOT block indirect uses of the power such as picking up a rock with the One Power and throwing it at the wearer. Air constraints, jets of fire, etc. aimed directly at the wearer are cancelled. The medallion also grows cold when someone is trying to channel at the wearer. Needless to say, Mat enjoys the perks.