When playing taijiquan, does the knee need to buckle toward the thigh root?

Whether the knees need to be buckled or not when playing taijiquan depends on the specific situation at the time, and it is not a one-size-fits-all rule.

When practicing taijiquan, the strength of the legs is divided into outward strength and inward strength. In the case of the leg that enters the force, the knee has the intention of closing in. Ensure that the hips are loose and the muscles at the back of the thighs are slightly externally rotated, so that the force can be rotated and twisted under the feet and rooted in the earth. For the leg that is out of strength, the knee is slightly outwardly braced to ensure that the strength comes up from the foot and pushes the crotch on the side, thus shifting the center of gravity to the direction of the leg that is in, and completing the reversal of the center of gravity.