Not unlike the planetary motion of the Earth itself. This gives you a clue about application of the motion. Crossing your center, your opponent's center, your limbs, his limbs, both of your limbs, etc. The kinetic power of Heng Chuan is imparted primarily from the waist and costal spaces. Direct vectoring of force is not possible in a spiral, so the role of the legs in this form is to assure solidity of the root at the moment of impact.
More solidity means more potential power.
You must be well anchored to ensure that none of the projected energy of the movement rebounds back into you, thus weakening the technique. For application of Heng Chuan, try to remember to be broad minded in your quest for viable techniques. Do not be one dimensional in your thinking. He often expounded on the fact that a change of hand position, angle, or footwork was necessary to adjust the technique to an ever changing situation of fighting. This is what makes Hsing I Chuan so complete.