This is a form of "meme busting" where you interrupt a person about to have their photo taken by a friend or their cam set to auto while they are planking. Most commonly this interruption is in the form of spanking that persons ass rather hard with with a paddle or other blunt object.
Throwing water on the person is also a hilarious way to "spank their plank".
It is important to remember to have a friend or your own cam set to auto to take a picture of this while you do it. It is very funny if the victim's own camera or friend taking their picture is also in he shot so you can capture their reaction as well.
There are also ways to plank spank that do not involve contact or soiling the victim such as (but not limited too):
Farting near them
Blowing an air horn
Yelling at them "memes are stupid!"
For more details on methods and finding memes to bust see: meme busting
I just got a great shot of me planking spanking a guy in central park.