The other day I watched a heron stand on a grassy field and eat five live baby gophers. It waited in stillness and then struck suddenly into the soil. I have it from a reliable source that they are actually feeling the vibration of the gopher under the ground. They must use the difference in the intensity of the vibration in each foot to determine the location of the gopher. They may even develop a mental picture of the gopher's movement and tunnel system.
When you practice internal martial arts you want your feet to be so relaxed that you could pick up even the slightest vibration and get a mental picture of what is causing it. Any tension or excess movement in your feet and this ability will be obstructed. Your feet must be completely devoid of an agenda.
Our great ape ancestors hunt gopher-like critters called bushbabies. They sharpen sticks and wait by their holes to spear them. My guess is that they also have some ability to sense vibration with their feet and use it to create a mental picture.
I have an "edge theory" based on my experience which goes like this. At the tip of our tail bone there is a small sphere called the coccygeal body (Wiki). It is surrounded by a capillary net strongly suggesting that it excretes something which goes directly into the blood, a property which would make it an endocrine gland. But so far, no one has figured out what it excretes. My theory is that vibrations come up both legs and meet at the tip of the tail bone simultaneously triggering this gland and vibrating the spine all the way up to the teeth. If the frequency of the vibration is one we associate with small animals which we would like to eat, our mouth starts watering. If the frequency of the vibration is very deep like from an elephant, a stampede, or a lion, our mouth goes dry, creating a fear response. (Here is a wacky site which presents another edge theory.)
Anyway, practice keeping your feet so relaxed that you can feel under the ground.