This is a sophisticated question. There is no one good definition to describe textual dynamic, because so much can be stated. In light of this is best simply to say that textual dynamic is whatever an author does to make his or work more creative.
Let me give you some examples of how this can look. If you look at Plato's Symposium, there is so much going on a literary level. We can say that there is so much textual dynamic.
First, we have a philosophical discourse in the form of a drinking party. this point alone is interesting to make, because as the work goes on, there is a sense of drunkenness. At the end of the work, everyone is drunk except for Socrates. So, this begs the question of whether what was said was reliable.
Second, in Socrates' discourse, there is the conflation of ideas. Love is combined with the idea of fertility. This makes see love as procreative - something to consider.
Third, there are many learned allusions to the religious myths of the times. For example, when one of the characters (Alcibiades) comes into the work wearing the traditional clothing of the god Dionysus, there is an intentional connection. And when Socrates is able to withstand Alcibiades, there is a hint that Socrates can conquer Dionysus.
Textual Dynamics: It's about how authors or creators of texts play around with the norm of telling a story.