The Cliff-notes version is that in black jack you have a slight statistical advantage since you know what cards have already been played. If there are lots of face cards left in the deck, then the dealer has a higher chance of busting. It's too small for a single player to make much money consistently (just a couple % advantage), but the MIT guys would have several people ("spotters") playing small hands, watching for a deck with lots of high cards left. They would signal to the "Big Players" who would come in like they owned the place, play $10K per hand and make a killing on the hot deck. Then when the cards ran out and were reshuffled, the Big Player would split and look for another hot deck at another table. This way they could multiply the gain when they found a hot deck.

"Bringing Down the House" (the book the movie was based on) was one of the best of the 100 or so books I've read in the last year.

PS How cool is it that Kevin Spacey reads Wired?