Body Rocking is a non-verbal false time constraint that a PUA can use to convey that he is not too needy and that he may leave the set at any time. An example of Body Rocking would be to slightly turn away from a set, as if you were just about to leave. This creates a feeling of loss in the target, and she wants him to stay. Body Rocking should be done smoothly and unconsciously as a bait to reach a hook point in a set, or make the girl give an IOI and chase after the PUA.
Additionally, Body Rocking sometimes refers to the unconscious fidgeting that nervous guys will exhibit in set, where they shift their weight back and forth from foot to foot. Such rocking is very poor body language and should be avoided as much as possible. In general, it is best to keep any unnecessary movements to a minimum in order to appear confident and relaxed.