Biofeedback is a method used to help people control biological functions such as heart rate, blood pressure, muscle tension, etc. It stems from the idea that the mind can control the body's activities, even the seemingly involuntary ones, with adequate training.

Patients are strapped to a machine that measures a certain biological function. The machine produces a visual representation of said function, and the patient attempts to alter the behavior of the visual representation, thus indirectly altering the biological function.

For example, my friend Bob, no relation to your friend Bob, has a unusually high heart rate. He decides to give biofeedback a try hoping to solve his malady. Bob gets himself a heart rate monitor that shows a graphical representation of his heart rate on a screen. Imagine one of those things you see in the movies that make that annoying beep beep beep noise, until the person they're monitoring bites it, after which they just make beeeeeeeeeeeeeeeeeeep noise until the nice nurse lady turns them off. They're called electrocardiographs (EKG's).

So Bob has himself an EKG and it's showing his heart rate. Beep. Beep. Beep. Blah. He's sitting in his favorite chair and he's looking at this screen. Beep. Beep. Beep. Beep. Bob's not quite sure how this is supposed to work, but someone told him it's like those cross-your-eyes-3d-puzzles. You just stare at the thing for a while and eventually you get it. Bob is a bit skeptical, but this high heart rate thing is pretty important. He's going to give this a serious try.

Beep. Beep. Beep. Beep. Beep... Nothing. He's looking at the screen but nothing's happening. It just beeps, and the little wavy lines go across the screen. It never changes.

Beep. Beep. Beep...

Bob thinks for a while and comes up with a plan. Instead of just looking at the thing, he's going to stare it down.

Bob: Intense "I'm gonna rip out your liver and feed it to you" look.
EKG: Beep. Beep. Beep. Beep. Beep.

Bob: Intense "Bow down for I am root" look.
EKG: Beep. Beep. Beep. Beep. Beep.

Bob: Intense "I know Kung Fu" look.
EKG: Beep. Beep. Beep. Beep.

Hmm... Perhaps something different...

Bob: Soft "I want to make love to you woman" look.
EKG: Beep. Beep. Beep. Beep.

Bob: Sly "Shaken, not stirred" look
EKG: Beep. Beep. Beep. Beep.

Bob: Stupid "I just shit my pants" look
EKG: Beep. Beep. Beep. Beep.

Bob: Gross "I just ate WHAT?" look
EKG: Beep. Beep. Beep. Beep.

ARRRRGGGGGGHHHHH! Bob is getting upset. Stupid machine. As Bob gets genocide angry he notices something.

Beep. Beep. Beep. Beep.

It's working. Bob thinks of mayhem, destruction and chaos.

Beep. Beep. Beep. Beep.

At last, Bob is pleased. He has successfully learned how to alter his heart rate. And there you have it. Biofeedback.