Originally Posted by Ed~
Even more crazy sounding but works in a desperate tank-slapper: stand on the pegs, lean forward, and loosen your grip!
Moving the weight distribution forward instantly changes the steering angle and ends that back-n-forth positive feedback energy on the front wheel to eventually allow it to regain its natural gyroscopic stability. There is no way your arms could realistically counter that alternating force (probably make it worse if you tried) so it is safer simply to loosen the grip and let the bike sort itself out. Finally, we all know that standing up when riding in sand or gravel helps to maintain control of the bike even when the entire bike is sliding and wallowing beneath us.
Tankslappers,especially those on tarmac, are the result of the bike and rider reaching a resonant oscillation in some part of the bike/rider system.This is why they often happen at a specific speed but lessen if you go faster or slower.The bike/rider system is like a whole bunch of springs and pendulums traveling down the highway at 80mph.You've got a flexible joint between the rider's center of gravity at around belly level and the bike's CoG near the engine.The tyres flex and the supension moves up and down.Also the steering moves and is linked by the rider's arms to the rider's CoG,but also to the bike's CoG,making it a very complex system.
The reason moving the rider's position,changing tyre air pressure,fixing worn suspension or moving/changing luggage stops tankslappers is because it changes the frequency of resonance.Loosening your grip on the bars reduces the coupling between the rider's weight and the steering mass and can kill the resonance.It's like how lengthening a pendulum's string will change the frequency of it's swings.
Another technique that can kill a tankslapper while it is happening is to drop your torso down onto the tank,this removes much of the flexible joint I mentioned previously, between the bike CoG and the rider CoG.