That's a line from a song I wrote, years ago, called Novocaine Blues. Of course, it refers to the apocryphal story of Sir Isaac Newton formulating the Law of Gravity after being bonked on the head by a falling apple. I'm a big fan of Newton. I'm glad that the aliens came down and fed him all of that information...
Newton's Third Law holds particular significance to bicyclists: For every action there is an equal and opposite reaction.
This is what causes a bike to move forward when you pedal, and what causes you to slow when you put on the brakes. It's also what causes an overly-padded bike seat to be uncomfortable.
Periodically, someone will look at the saddle on whatever bike I am riding and declare that there is no way that the unpadded leather seat could be comfortable. "I would need on of those big padded seats," they invariably conclude.
I try to explain to them that the padding actually causes pain, after a short while. When you first get on one those miniature sofa cushions, it does seem comfy. But, this changes after a while.
When you sit on the padding, it compresses, and stays compressed as long as your weigh bears on it. True to Newton's third law, though, the padding is pushing against you with a force equal to that which is compressing it. If you weigh 150 pounds, you probably are (at a guess) putting about 100 pounds of force on the saddle, in cruising mode. The rest of your weight is supported by your hands on the handlebars and your feet on the pedals. Of course, the amount supported by your feet increases if you sprint in the saddle (and it increases a lot if you stand to climb or sprint).
But, just for illustration, let's stick with 100 pounds of force on the saddle, compressing the padding.
That means that the saddle is pushing back against your bum with 100 pounds, or so, of force. Soon, hot spots make themselves known, and you are forced to shift your weight to relieve them. Then, they appear again, at another point, etc.
With an unpadded saddle, there is no compression of foam rubber to generate the reflective force. Your weigh is borne by a flexible, but non-compressive, surface. Pressure exists, but it is direct pressure, not a re-directed Newtonian kick in the ass.
Now, I'm no physicist, nor a physiologist. Perhaps my understanding of the forces in this instance is flawed. If so, and you have a better explanation as to why I can ride all day on a Brooks but I can barely manage 50 miles on a padded saddle, let me know.
For now, I will go with this explanation.