The referenced essay below by Thomas Dyhr is long and rambling, and also a bit arcane and mysterious in places. In reality I don't know that it does any better at explaining Hunyuan, but it offers a different take on it. In that light I share it:
"Hunyuan is a Daoist term which is difficult to translate, but which can best be imagined as the nucleus of a human cell. The chromatin, the genetic code which moves chaotically, that is spontaneously, is like prenatal chaos – the “primary mass”, the cosmic primordial ocean. In this prenatal and chaotic state the chromosomes are invisible, intangible. When cells begin to divide themselves, two energetic poles are created, and the previous “chaotic” chromatin begins to spiral toward the poles until the poles have gathered enough energy to produce a single cell. Hunyuan depicts this moment where the primary mass, or the “absolute highest” (Taiji), creates the polarity and appears in various forms only to ultimately return once again to the whole. Hunyuan is the sacred process of the creation of a form which is created from the void. Thus, what is typical of Hunyuan is that these processes flow in an extraordinarily circular manner." The Rest