I hate to be a pest, but…
I want to be really really clear about what I mean by the “ball of the foot.” So, even if you are certain that you know what I mean, please read this carefully. At the very least, it may help you explain it to someone else.
In recent seminars, I described it, pointed to it, even had people take off their shoes as I touched the spot…and they still missed it. Even students who have been with me for decades! For some, it was a matter of having done it differently for so long that this new information just didn’t register. “Oh yeah, I know that.” Others just didn’t believe that something so simple could have such immediate power.
But it does. As one email I just received said, “The sesamoid bones are the sweet spot of the sweet spot. Nothing launches you more precisely into the exact arising moment.” (Jonathan Bricklin)
Since there is some abiguity about the exact location of the “ball of the foot,” I have drawn a picture of what I mean by the term.

Medial Sesamoids Bullseye
Weight is distributed throughout the foot, but this is the bullseye. Center the body’s mass here by setting the knee. It’s that big knuckle on the first metatarsal bone (on the big toe line). There are two sesamoid bones embedded in a tendon at the bottom of that bone, where it meets the floor. (In anatomy, a sesamoid is a bone embedded in a tendon or a muscle. The kneecap, or patella, is an example of a large sesamoid bone.) Ordinarily, there are two sesamoid bones on the first metatarsal bone, called metatarsophalangeal sesamoids. (Don’t worry, this will not be on your mid-term exam.)
Here’s a diagram from Wikipedia:

Metatarsophalangeal sesamoids
The white arrow points to the space between the sesamoids. It is fitting that bullseye is an empty space, since the “sweet spot” is an insubstantiality, like the empty space at the hub that holds Lao-tse’s “thirty spokes” of the wheel:
“Thirty spokes converge at the hub,
but emptiness completes the wheel.
Clay is shaped to make a pot,
and what’s useful is its emptiness.
Carve fine doors and windows,
but the room is useful in its emptiness.
What is,
is beneficial,
while what is not
also proves useful.”
In Western Gate, I wrote about William Chen’s “Three Nails.” This is the middle “nail.”
This nail organizes and anchors the other two, and we engage it by setting the knee in the optimal position. Optimal is determined by its effect on the energetic connection. That is, the physical relationship is established by setting the knee to find the sweet spot. We use the knee to establish the relationship of the body to the earth and sky through the ball of the foot, and that unlocks the Big Qi. The relationship must be renegotiated moment by moment, as the body moves through space. How much of the Big Qi we can tap at any given moment is greatly affected by how much we consciously connect to the earth through the ball of the foot.
The dot in this illustration indicates the location of the “Bubbling Spring,” Kidney 1, the primary gate to energetically contacting the earth qi. This energy gate opens when we plug in through the ball of the foot. At times, I have considered the Bubbling Spring to be the actual anchor point, but after testing it on hundreds of people I have been disabused of this idea. Test it using a neutral stance. If you’re not centered over the ball of the foot, you are easily uprooted.
Notice that Charles’ body mass is centered in his heels, so that a much smaller Maria can easily uproot him. Central equilibrium (zhong ding) is most easily established by centering over the balls of the feet.
If you set the knee correctly, you can remain rooted and withstand considerable force. But equally important is that it gives you immediate access to the “soft power” of taijiquan, jin. Jin is contrasted with li, or crude muscular force. Li is generated by muscular contraction. Jin is energy that is controlled by conscious intention and expressed through the connective tissue system. In this photo, Stan demonstrates jin as the combined muscular force of Dennis and Charles is easily neutralized.

Effortless Power
When we plug into the “Big Qi,” we are no longer limited to the energy of our own bodies. We become part of a much bigger system, a virtually infinite energy source. The only limit then is our how much qi we can comfortably wield.
And that is determined by our gongfu.