When the fuel in a vehicle hits the inside of the tank hard enough to shake it, nearly always cause by making sudden, unplanned turns. Tank slapping is a symptom of bad driving.
Many cars use special baffles to help reduce this.
Can also include the rider's legs on a motorcycle.
Leaving the last bad corner on Snake Pass I missed the apex and over corrected. When the tank started slapping I thought I was a dead man.