Normally, when you're riding your bike, the downward vector of gravity on either side of you cancels itself out, because you're balancing while you ride. But when you tip one way or the other, that downward pull becomes much more important than your forward motion, and you fall down. That can hurt your body, which is why we wear our pads and helmets, but if you think about how momentum works, you can see that the force acting on your motorcycle is even more intense: Your mass is nothing compared to the bike's, so your momentum is that much less.
It's the action of these two vectors -- the friction between your momentum and gravity -- that grinds your bike against the road, until the forward motion finally comes to a stop. The vectors are using the physical material of your bike to work out a simple physics problem about which one of them will win. The slower you're going when this happens, the less damage will be done, because the vectors will equal themselves out more quickly.
Likewise, the longer the process takes, the safer your bike will be. Imagine dropping a dinner glass onto the ground, versus skipping it down your driveway. Those bounces represent sudden sharp adjustments to this vector ratio, and while they're dangerous, they certainly don't create as much friction -- or as much downward force -- as simply dropping the glass. But if you imagine rolling or sliding the glass, you can easily see how the danger is comparatively minimal.
What a frame slider does is help prolong the process this same way, through destroying the puck and absorbing the force as designed, but also by lubricating your bike's contact with the road and distributing the force of impact outward to the rest of the frame. In all cases, the physics remain the same: Your frame slider is lessening the friction between the two vectors and keeping the sensitive mechanics inside your engine from being ground against the road.
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