It's about some integral similarities between 4's and 6's. It deals with the cognitive awareness [T] - intellectual, inner knowing; the emotional awareness [F] - emotional, sense of self; and the motivation [M] - acting, implementing. Essentially, it contrasts the dynamics of each of these facets within the 4 and 6 personalities.
The 6 is cut off from their inner knowing, while consistently residing in their sense of self [F], which is based on feelings of fear, loyalty, rebellion, etc., due to the separation from internal guidance. In order to reconcile this, they act based on these feelings [M], substituting external things to compensate for the lack of inner knowing.6 Centers: [T] F⇒M
The 4 naturally resides in their emotional sense of self [F], but their is a block on it (not knowing the "true" self), so they use their intellect [T] to fuel the process of idealizing and cultivating emotions [F] in an attempt to attain this. They are not cut off from their inner knowing [T]; in fact they are too in touch with it. Their desire to act on their sense of self or knowing [M] is generally disregarded, unless it facilitates their sense of self. They tend to simply react based on these things, which is typically unproductive.4 Centers: T⇒F M
What's interesting is, this type of diagram could probably be applied to all the other types. For example, the 7 is cut off from their feelings, and uses their thinking to fuel their motivations, so as to avoid feeling empty and give the illusion of security -- [F] T⇒M. And so on...