So I came Across with this New word by watching one of the videos here talking about tank slapper, since I'm new to motorcycling i went and research. came across with this explanation by urban dictionary.com
tank slapper
When a motorcycle shakes its head. The front forks going from lock to lock.
Sen. He got into a big tank slapper and stepped off on the high side.
Now i went to Youtube And Got This Amazing videos
Link to original page on YouTube.
Got Scared when i saw this Vid, i kept searching and i think this is the best way to describe a Tank Slapper 
Hope you Enjoy this 2 videos
Link to original page on YouTube.
i had to watch the vid over and over again At 3:38 