Because of surgery a few years ago cutting out a saliva gland, I have to chew gum to function effectively. I had never done gum before and very much dislike it now, but I do have some "haptic' felt sense of what they are talking about, how it combats "vigilance decrementia." It at least gives me something to do during interminable harangues during faculty meetings.
My guess, however, is that it has something to do with keeping the wiring that goes from the brain to the articulatory equipment energized, in effect working in the opposite direction, very much like haptic technology drives feedback back to the brain through the hands. Not sure I'm in for having students do gum during work that is basically oral production-oriented, but next time your class has to just sit and do nothing but listen, give it a try. "Gum up the work a bit, eh!"
Journal reference (compliments of Science Daily): Kate Morgan, Andrew J. Johnson and Christopher Miles. Chewing gum moderates the vigilance decrement.British Journal of Psychology, 8 MAR 2013.