Taijiquan consists of countless circles. It is a violation of the principle of Taijiquan if the circle is not adhered to, even in lifting a hand or picking up a foot. Half the circle fulfils the role of defence, the other half that of attack. The circle becomes smaller as the gongfu becomes deeper. Often a circle is not visible to detect attack and defence.
Tuishou is also a Taiji circle. Within the circle there are peng, lu, ji, an (warding, diverting, squeezing, pressing), peng, lu taking half the circle and ji, an taking the other half. At the moment of contact with a partner, each hand takes on a circle, half of which sticks whilst the other follows. To be unclear on the roles of the two halves of a circle is to become double-weighted.
