One misunderstands the body and mind's characteristic of totality and thus grasps the self of the person as an actual existent.
What is this "characteristic of totality"? Their apparent unity?
The characteristic of totality is one of six characteristics (in Chinese known as liu xiang
六相). They are as follows:
①総相 - Totality
⑤成相 - Integration
⑥壊相 - Disintegration
There are a lot of specific terms and ideas that Huayan thinkers crafted and applied to their practice. The big one is the relationship between principle li/理 and phenomenon shi/事. The former being the truth or ultimate and the later being the conventional reality of "things". True and false are always dependent on each other and one can never speak of one without the other.
To put it into modern day terms, the Huayan thinkers stressed that everything is exists in a relative sense. Unlike the methods of Nagarjuna which stressed that things are ultimately unarisen, the Huayan line of reasoning didn't generally take the analysis that far. They were content with seeing the relative mode of existence of things, which is to say things being infinitely interconnected and relative to one another on an inconceivable scale, as sufficient for their purposes.
《註華嚴法界觀門》卷1：「除事法界也。事不獨立故。法界宗中無孤單法故。若獨觀之。即事情計之境。非觀智之境故。」(CBETA, T45, no. 1884, p. 684, c4-6)
"Except for the dharmadhatu of phenomena, phenomena are not independently established and so within the dharma-dhatu school [aka Huayan]" there are no isolated or singular dharmas. If one observes something on its own then this phenomenon would be an object of conventional perception because it is not an object seen with wisdom."
One might accuse them of misunderstanding Indian Madhyamaka, but I feel that would be inappropriate because they were aware of it and had a specific classification for that view.
《華嚴一乘教義分齊章》卷4：「答八不據遮六義約表。又八不約反情理自顯。六義據顯理情自亡。」(CBETA, T45, no. 1866, p. 502, c4-6)
"Answered: 'The eight negations are based on negation methods [literally obstructing/concealment] and the six meanings are based on methods of revealing. Again, the eight negations reject the conventional and the principle reveals itself. The six meanings rely on revealing the principle and having the conventional itself disappear."
'Six meanings' is synonymous with the six characteristics above.
Now, if should apply this analysis on the ground in a practical sense, we might consider Fazang's analysis.
The object that Fazang took into consideration was a building. In simple terms:
Totality is the whole building itself.
Partiality are the various individual conditions (rafters, beams, walls, etc...).
Similarity is the aspect where everything is identical on account of being dependently originated ergo empty. Everything is identical in the sense that everything is empty.
Difference is the differences one observes from the standpoint of the individual conditions.
Integration is the aspect where the individual conditions create the result.
Disintegration is the aspect where each condition maintains its own individual characteristic.
Totality and partiality are the essence 體.
Similarity and difference are the characteristics 相.
Integration and disintegration are the function 用.
Finally, all these meanings or characteristics include the others. This is where we go from the concrete to the entirely abstract. Essentially these aspects are all relative to one another and each lack any sort of intrinsic identity within themselves.
I think this is not at all unlike Nagarjuna declaring that even emptiness is empty. These six meanings as well in the end vanish under the force of their own analysis.
Unlike the Madhyamaka method which uses thorough negation to reveal the lack of intrinsic existence within an object, thus concluding that said object does not ultimately exist and is unarisen, the Huayan method uses a kind of affirmative method to show that an object exists entirely dependently thereby by necessity not having any kind of intrinsic existence at all.
From the Huayan perspective the saying in the Heart Sutra, "Form is emptiness, emptiness is form," would mean that form can only exist because it is empty -- in other words, because its mode of existence is dependent origination -- and emptiness can only be because there is something that could be empty.
In short, for the Huayan thinker everything without exception is relational. When you apply that analysis to your own individual being, which seems to intrinsically exist as Nagarjuna might put it, you discover that you're neither really existent nor non-existent, but infinitely interconnected with the rest of the universe. You're but a jewel in Indra's net being reflected by all the other infinite jewels and you yourself are reflecting all those other infinite jewels. Your sense of "me" and "mine" vanishes as your sense of "self" expands to include all of totality and suddenly all those other infinite sentient beings are a part of your identity. At that point you're like mother and child. They're all drowning in the sea of samsara. You don't think twice about diving in and rescuing them. Your Bodhisattva vows go into high gear.
Hence emptiness and compassion are non-dual.
Huayan Master Fazang once wrote, "Seeing that form is emptiness manifests great wisdom and one does not abide in samsara. Seeing that emptiness is form manifests great compassion and one does not abide in nirvana. When form and emptiness are non-dual, compassion and wisdom are not different."