Spread. To spread means that we mobilize our chi spread it over our opponents energy and prevent him from moving.
Cover. To cover means that we use our chi to cover our opponents thrust.
Check. To check means that we use chi to check our opponents thrust, ascertain his aim and evade it.
Swallow. To swallow means that we use chi to swallow everything and neutralize.
These four character transmission represents what has no form and no sound. Without the ability to interpret energy and training to the highest perfection, they cannot be understood. We are speaking here exclusively of chi. Only if one correctly cultivates the chi and does not damage it, can one project it to the limbs. The effect of this on the limbs cannot be described in words.
(attributed to Wu Yü-hsiang)
Reference:
Tai Chi Touchstones: Yang Family Secret Transmissions by Douglas Wile
Sweet Chi Press, April 1989
ISBN: 091205901X
Page: 27