When landing on your foot or feet, you compress tendons, ligaments, muscles. If you return to original shape quickly enough, the stored energy in the springier elements in this kinetic chain give you an extra push. Even when running slowly, pop your feet up quickly and deliberately to take advantage of this.
To test how quick, deliberate motions take less effort, try jumping an imaginary rope at a natural rhythm for 10 jumps. Then try to jump once every 2 seconds for 10 jumps. Which took less energy?Your natural jump rope rhythm and rate-of-bounce turns on an amazing kinetic chain that returns much of the landing energy (ground force reaction) to the next jump. To turn off segments of that chain try this.
10 jumps on your imaginary jump rope without bending your knees (turns off quad and hamstring absorption). Reset. 10 jumps on your imaginary rope landing on your heels (turns off lower leg, or gastroc. chain including achilles tendon, calf, soleus, etc).Now to the cushioning. An extreme example of cushioning would be jumping on a trampoline. If you tried jumping rope on a trampoline, you'd find it's ~3 seconds per hop. You are adjusting to the resonance of the trampoline's elasticity. You proved your natural rhythm is much faster, but you had to move to an unnatural rate to accommodate an outside spring put in series with your built-in springs.
A less dramatic example would be hold a marble 24" from a hard counter top surface and let it bounce. notice its first bounce is only a few inches less than 24. Now place a paper towel on that counter and repeat. You've lost 50% of the bounce. The marble deforms more and thus returns more energy without the 'help' of the thin cushion. Try a thicker cushion, try something springy - anything make bounce higher than the original test? The poor deformed marble needs to re-expand to its original shape to deform this new cushion THEN push itself back up. These examples are of simple systems, elasticities that are rigid and only one value. The beauty of the human form is muscles can tighten and relax in a way to adjust the elastic properties of your kinetic chain. If you've ever ran across a trampoline, the trick is to pop your foot up so fast that you don't have time to sink into the trampolines large elastic value. Pop-pop-pop. That actually takes more energy, but you compensated. Your shoes' cushions are the same way. You can compensate, change your natural rate, vary your leg's spring-compliance but why should you? Why are treadmills another layer of springiness you didn't ask for?
My first minimal shoes were slabs of thin, hard rubber with no foam and no EVA (also... no spring, rockers, air pockets, etc). It took some getting used to but letting the natural springs take over took me from 5km to 80km runs.
Beware of those unintended consequences. Remember that 'help' isn't always helpful.
