Rooting is an important practice is tai-chi and other forms of internal martial arts. In the past, students had to go through a period of rooting before proceeding to learn combat techniques.
What is rooting then? The first stage of rooting is actually zhan zhuang. And the criterion of success is "muscles as one" (肌肉如一). According to I-style, there are two kinds of zhan zhuang: health form and combat form.
The second stage of rooting is actually a conditioning for blocking or preventing being pulled/pushed over (and as a minor objective, conditioning for receiving strikes 挨打). Here one needs a partner. Essentially it is for a student to stand in either health or combat form with the partner gently applying force at the right point. It is important that firstly the force must be gentle, and secondly the student must direct his counter force from his feet.
This is a very ingenious method to condition and train up one's body structure. I shall go into details in future post.
PS: One interesting question is: How do Muay Thai practitioners build up their structure? I shall do a comparison some day.