Apparently it is easy to do with some sort of morse code or code familar to the player. From this incident it sounds like he may have had a device under his armpit or on his back that would give a certain vibration code to tell him where to move. Apparently, he would have to input his opponents move by entering some sort of morse code on a device located somewhere on his body (perhaps leg) that would tell the system where his opponent moved and then it would send it to a chess engine (Houdini) which would vibrate and tell him where to move next...ie..long vibrate-short vibrate-long vibrate=move Knight to D5 or whatever he had set up. Also, according to some articles the improvement in Ivanov's rating is unheard of.
http://www.cse.buffalo.edu/~regan/ch...and-report.pdf--according to this study the odds of a player rated like Ivanov to reproduce engine moves to be roughly 1 in one million.
Also, here is a little history on others cheating and what they used.
Cheating in chess - Wikipedia, the free encyclopedia