I've read about guys experiencing head shake and Tank slap and was wondering if someone can shed some light on what exactly it is, why it happens, how to react and how it can be prevented

 

I thought this was about a new dance move
...plus it's the wrong place to post it FYI.

Tank slap, an example of harmonics, is a dangerous and scary situation condition for motorcycle riders.
With motorcycles being light, it is very easy for the front end of the motorcycle to lose contact with the road surface. When the front end makes contact again with the road surface and is not in line with the back tire, the front end reacts violently transferring the energy into oscillations of the handlebars.
It is recommended to maintain throttle position and let the oscillations diminish. The reason for this is to allow the motorcycle weight to reach equilibrium. Applying the front brakes or chopping the throttle, puts a forward bias of the front end which combined with oscillations, leads to a break in traction causing an accident.
Retrieved from "http://en.wikipedia.org/wiki/Tank_slap"