It involves lying down on your stomach in a public place, with your arms on your side and feet pointed, trying to resemble a plank.
"Plankers" will often take photographs of themselves and post them on facebook, so that their other planking friends can see what an amazing accomplishment they have achieved.
These are the official rules of planking according to there Facebook page:
When performing a Plank:
1/ You must always lay face down, ensuring your face remains expressionless for the duration of the Plank.
2/ Your legs must remain straight, and together with toes pointed.
3/ Your arms must be placed by your side, held straight and fingers pointed.
4/ You must make it known that you are Planking. Saying 'I am Planking' usually get this across. Sternly announcing it will ensure a good result.
5/ Your safety should always be considered. Properly thought through Planking procedures should always go to plan. Never put your self at undue risk.
6/ Every Plank that is captured must be named.
John "What? You do that gay shit? Get away from me"
Planking is a great pastime for people who get bored easily and have friends willing to take pictures of them looking like retards. Public planking is more adventurous and is harder to pull off especially on things like: cop cars, public toilets, and in the middle of six way highways.
Planking can also be dangerous if performed: in places of frequent use, the tops of tall buildings and placed frequented by pedophiles and rapists.
Sexy chick : "Nice! I am spastic eagle planking!"
Dude : "You gotta teach me that one"
Proof that literally ANYTHING can catch on if enough idiots think it is "cool".
Hipster 2: "DUDE! I was planking on some benches at the quad yesterday!"
Normal person: "You guys are fucking stupid."