For those of us who were unsure what it meant, Monte Cook's mentioned it in an interview on MTV:
For the Game Master, you aren't necessarily so bound by hard-and-fast rules. In fact, there is a rule called GM Intrusion. GM Intrusion is when the Game Master decides to make a change in an encounter to make it cooler. At any point, the Game Master can say, 'GM Intrusion! As you are fighting this weird beast, your sword falls out of your hands and goes across the floor. Now what you going to do?' And a player has to deal with a new circumstance, but on the flip side the player effected gets an Experience Point. (Unlike in D&D, one Experience Point is pretty significant in Numenera.)