Tai chi was invented by the famous Taoist Zhang Sanfeng (張三丰). Every year tai-chi associations or groups that still teach their practices the traditional way celebrate the Grandmaster's birthday (張三丰祖師誕). Because of the sexual symbolism of Taoist yoga/meditative practice, martial artists of tai-chi tradition oftentimes distant themselves from the Taoist. And some keep their umbilical cord with Taoist yoga by keeping certain mysterious practice for an inner circle of faithful and devoted students. Like the subject of archeology, some remnants of this connection can still be found in what practitioners call "reviewed practice", or generally accepted guidelines for sound tai-chi practice.
One of this guidelines is "Use mind but not use force" (用意不用力）, one of the Ten Essentials of a prominent Yang style master - the late Yang Cheng Fu （楊澄甫） - Essential number 6.
In Taoist yoga classics (for example, Hui Ming Jing (慧命經), Fire (a metaphor) was said to be created by the mind, in particular, through point-meditation. How can the Fire (or chi) travel? It is through the mind's directional connection of two energized (meditated) points. The end result of this meditative or mind technique is a transmission of chi or chi-force. When this mechanism is used in Martial art like tai-chi, bodily movement is synchronized with this meditative or mind technique, resulting in a transmission of physical force together with chi-force.
The only literature on this mechanism that has been explained (,) is a book in Chinese called "Wu-sytle tai-chi (point-line movement and join-circular movement" by Master Huang DiXian (吳式太極拳：走點線和走貫串 －汪棣賢). Interested reader can refer to the book (PS: I don't personally know Master Huang, so no commercial intended here!)