The Chu-style Wing Chun emphasises “rotation in joint” (refer to the last post dated 2013.11.29). Strictly speaking, it differs from the generic description “joint rotation”, in the way that it stands out the requirement of initiating the rotation within the joint structure itself, unreliant on any structure outside. This is an important characteristic that yields the remarkable new way of moving enabled by Idea: “joints rotating separately but at the same time”.
Take Wing-arc Hand (Bong Sau) as an example. First, the Wing-arc Hand is not “raised” up; it is “rotated” into the desired shape. Second, in the mind the entire arm is not regarded as one thing to rotate; it is treated as separate segments/parts “broken” at joints along the arm up to the fingers (see the remark below). Third, each of the joints (mostly referring to the shoulder, elbow and wrist joints, although they should of course include the joints in the fingers) rotates inside on its own (Idea reaches and acts upon right there), separately and independent of each other, but under the orchestration by Idea to start and stop at the same time and at the same pace in the respective desired (mostly different) directions.
You can say that the joints in the arm together have “composed” the Wing-arc shape movement by “rotating separately but at the same time”, in contrast to, again, raising up the Wing-arc Hand as a whole.
A remark here: the conception that the mind regards the arm as separate segments/parts is particularly true when you are on the way of acquiring this new operation. Once acquired and well-versed in, the mind is free to treat the arm as one, and further as none, which is already operating in the new way.