You shall not add to the word that I command you, nor shall you subtract from it.
Be’er Mayim Chaim: Chazal teach that adding and subtracting include performing a particular mitzvah by adding to or subtracting from the required elements in the commandment. For example, subtracting from the mitzvah of tefillin does not mean ignoring it. That would simply leave the person without the benefit of fulfilling that mitzvah. Rather, it means donning tefillin with only three sections of Torah text, rather than the requisite four. One who does so not only misses the opportunity to perform the mitzvah, but has violated a prohibition to boot.
Why, though, should this be? Why is a mitzvah improperly executed worse than not performing it at all?
We should be able to grasp some of what is behind this pasuk if we reflect on one quality of mitzvos in general. In kabbalistic understanding, every mitzvah is accompanied by an ohr, a “light,” or a particular kind of spirituality radiating from Hashem’s pure essence. Our growth, our connection with HKBH, comes from receiving and incorporating that ohr. Like each yom tov, each mitzvah offers a different ohr to the person who performs the mitzvah with which it is associated.
While this may strike you as straightforward and uncomplicated, it certainly is not. In fact, at its root it is incomprehensible. Just like we cannot grasp the essence of the Divine, so too we cannot really process anything that emanates from Him. Whatever this ohr is, we would imagine that we simply cannot fathom it, experience it, or gain from it. How can the finite encompass the Infinite?
While perfectly understanding how this is so evades us, we know that somehow, this is precisely what happens. Hashem gave us the ability to absorb each ohr, and be elevated by it. By its nature within Hashem, the ohr is so “bright” that it would overwhelm any lesser being. In order to be useful to us, the ohr must be partially hidden by various filters as it passes through the spiritual olamos / worlds to reach us. When it does, it is in a form that can interact with our neshamos on the level that they find themselves, attached to this physical world.
There’s the sticking point. The oros must arrive in the exact measure necessary to be held by the vessels that are available to receiving and holding them. Those measurements are reflected in the basic contours of a mitzvah. The nature of these oros determines that there be four parshios in tefillin, two in a mezuzah, four groups of tzitzis on a garment, and three lines of berachah in the blessing by Kohanim.
When we change the numbers, more happens than simply the failure of the mitzvah. The back story is that in the attempt to do a mitzvah, Hashem has matters arranged so that the attendant ohr is made available to us. If we add on to the mitzvah, the ohr is increased. The surfeit of ohr, however, overwhelms the receiving mechanism. The ohr then cannot be contained by the waiting vessel, and returns to its source. The recipient, however, is left in worse shape than before making the ill-fated mitzvah attempt. The mere encounter with the ohr leaves a mark, a trace of the ohr that struck it momentarily. This residue becomes fair game for the chitzonim/ negative forces that surround us, attempting to sustain themselves from the holiness of the oros. Had the ohr found a proper recipient, those forces would have been subdued. Since all that remains is this trace amount of ohr, the chitzonim and kelipos prevail
The result is the return of the oros to higher worlds, while the intended recipient not only fails to be elevated, but sinks lower than before. It is weighted by the negative forces that attached themselves to the faint afterglow of the oros that came, only to have to return.
Similarly, one who “subtracts” from a mitzvah essentially undercapitalizes the mitzvah. The ohr that arrives is weak, diffuse. The chitzonim/ negative forces are attracted to the spirituality of the ohr. Had the ohr been present in the strength that it was intended, they would be subdued. Having been dimmed, the chitzonim “feast” on it, without being repelled. Here as well, the recipient is dragged down to a lower spiritual level, to a place more compromised than had he not attempted a mitzvah at all.
The net loss in both cases is the reason why the Torah treats adding or subtracting to a mitzvah more harshly than simply failing to observe it at all.
 Based on Be’er Mayim Chaim, Devarim 4: 2
 It is crucial to understand that when we employ words like “pass through,” “receiving,” “holding” and others, we use figurative language. These verbs are not meant literally – nor could they, in dealing with spiritual concepts that do not work within our spatio-temporal world. We use words that are familiar to us to poorly approximate what we can somewhat faintly grasp about spiritual processes.