"Tank slappers" are usually associated with severe "speed wobbles". A speed wobble occurs at high speeds when the gyroscopic forces of the two wheels become unbalanced and cause the motorcycle to oscillate/wobble from side to side. This isn't what you want! With severe oscillations the front wheel may bang from stop to stop, hence the name "tank slapper". As mentioned before, many things can cause this unsettling condition to occur - but it usually is brought on by excess speed. I hadn't heard of it occurring under severe braking, although what was originally described sounded like a "high-side" crash, which would occur when the front tire locks and yanks the wheel over to the stop, pitching the rider off the "high side". High-sided crashes usually occur when a turn is also involved.
Back to speed wobbles: They are very scary and potentially dangerous. Their main cause is an imbalance which can be the result of one of many things - wheels, tires, bent forks (front or rear) alignment, or even wind pressure at high speed. As I said, the condition is usually initiated at high speeds and it will often start slowly (but not always) as the wobble progresses to a full "tank slapper" (if you are still holding on and upright). If a speed wobble does start to occur, no amount of force can "hold" the bars in a straight line - trust me, I know. That means you can not hold the bars in a straight line no matter how hard you try, and also no damper is that strong! You also do not want to make any sudden alteration or force to the moving vehicle - e.g. braking or quickly rolling off the throttle, as this will just make the wobble worse, likely immediately causing a "tank slapper". What you are advised to do is gently ease off the throttle while riding out the wobble as the speed decreases to the point of stability again. (Once this stability returns, you probably want to come to a complete stop so that you can go "wipe yourself".)
I have a 1962 CB77 that always developed a speed wobble at about 85-90 mph. I never dumped it but I had to wipe myself several times as I tried to diagnose and cure the source of the problem. My rear swing arm had a kink in it from the PO, which I had tried to re-align. Finally, after about 40 years of my analysis/procrastination, I replaced this swing arm with another, and the bike seems to track much better now - except I haven't had the nerve to do a complete retest of the situation. My front forks also seem a little "tweaked" so I don't know if it really is lining-up correctly yet.
One final word about "speed wobbles": Everything I've said so far applies to the street and should ideally be avoided for safety. However, on the track other conditions occur and apply. Race bikes today (and a lot of super sport "street bikes" are really the same) have very quick steering response, made possible by their front-end setups and steering geometries. They turn into corners with a slight flick and immediately snap to a lean - great for high-speed track negotiation, but usually referred-to as being "twitchy". When going fast, these bikes may hit slight track irregularities or get going up a slight incline at speed so that more down force is exerted on the front wheel, causing it to "twitch" or wobble. This is also called a "speed wobble", and the correct cure for it - on the track - is to apply more throttle and speed up. This "unloads" the front wheel momentarily, allowing it to stabilize again. If the brakes were applied, besides losing racing time, the front would load more and the condition worsen - likely leading to a "tank slapper" and a "high side". Race bikes (and a lot of today's sport bikes in general) are able to accelerate in these conditions, because of the immense power they are capable of generating - but on the street you are probably already going way too fast if this condition occurs. Besides, a lot of bikes riding the street don't have any more reserve of power to apply at these speeds so as to unload the front.
Sorry for the long rant, but this is safety subject that is a hot-button of mine. (There are no "dumb questions", especially ones involving safety.)
