When we speak of Si, we're referring to the degree to which a systems work manages to hold it together. Because work is Te, Fi the driving force behind Si's processes. It's very difficult to take the submicroscopic world as an example here, so instead I'll go one level up and consider things at the cellular level. (this will be complex, but only in complex situations can you compare the rational to the irrational. Perhaps in the context of this "complex" explanation, it will be feasible for others here to identify simpler examples.)
Si only appears in self-perpetuating, "homeostatic" systems. The question is of how the energies of the system cooperate to keep the system going, and to what degree. In the case of a primitive cell (we're discarding almost all talk of organelles and the like here) you have an organizing principle in the cell's RNA (DNA was a later development); a replenishing principle in the phenomenon of osmosis and in the permeable membrane; and an intradynamic principle in the relation of RNA to ribosomes and the creation of polypeptides. [note: I need to check my sources on this last bit; HS biology was some years ago.]
Immediately what appears lacking from the situation is Se; indeed, we can look upon the problems which plague modern cellular biology -- which is the traditional domain of the LII due to their fixation on (Fe) life -- as a direct consequence of the LII's difficulties with Se; indeed, one could even go so far as to reckon SLE George W. Bush's executive regulation of stem cell research as a macrosupervision of INTj-dom. The LII's hands are tied when Se enters the picture... or are they? LII problems of understanding the relationship is Se to Ni are equally daunting. (for example, researchers thought they had created a way to create "stealth" liposomes that could evade and provide superior drug treatment, only to find that the stealth factor diminished the effectiveness of the drug.)
From the LII point of view, Fi is begets Se. ESI agrees and understands exactly how best to manage the relationship. Recalling that Te is work and that Te is always seeking the best Fi, it follows that everything would be headed directly to its complement. This is not necessarily the case, because this simplistic reading of phenomena disregards the presence of Se "intent ". From where does intent come? Any polarized compound, no matter how complex, is seeking the ability to share electrons (work) in a way that does not reduce its self-integrity. Although Ni suggests that duality between two polarized compounds is ultimately preferred, it is irrelevant. This is the basis of the benefactor-beneficiary relation: the benefactor gives the beneficiary everything they want right away, and although the benefactor does not need they do not object to receiving it. Although socion duality is ultimately preferable, element duality suffices to the seeking party.
In this imperfect, socion-driven cosmos therefore, we have many "seekers" and "receivers", polarized bodies floating through space in search of a match. What if you had a compound that could not only manage its own energy, but project it outward to redirect other's effort? Then you would have an entity capable of willing its own search for harmony on its own terms.
Let's say organism A is just such an entity. Around organism A are organisms B, C, and D. Organism D is just perfectly matched for organism A, and organism A "senses" this by means of a chemical exchange medium shared by all four organisms. However, organism B is somewhat matched to organism A, and organism C is somewhat matched to organism D. Organism A wants organism D because D's chemical output is exactly what A needs to keep good Si; some of what B offers, in contrast, is useless to A and weakens its Si. As A's Si falls from interaction with B, stress functions of A are activated which change A's behavior. A begins signaling B with C's chemical, however when B responds with its own chemical A shuts it out and responds with a chemical irrelevant to B. Meanwhile A is also producing B's chemical for C, and shutting out C's response chemical. A's chemicals to B and C, meanwhile, are causing B and C to create chemicals for each other. C is no longer making chemicals for D, meaning that if B and C both are limited to creating one chemical at a time, A can offer its chemicals exclusively to D. A has, by means of reorganizational "prowess", taken control of the "situation" by satiating the obstacle Fi of B and C while increasing its own Te toward its desired Fi attractor.
(I know that was a tortured example, but I'm grasping at straws here.)
Basically, Se is the degree to which a compound is organized to reach its polar complement given the surrounding polar environment. The better it can manage its environment, the better the Se and by extension, the better the Si it can manage for itself.
Another point: Se is a purely relative function because it is irrational. We say it can manage its environment, sure... but relative to what? Obviously Se only goes as far as it can clear out that which obstructs ITS goals, and it ALWAYS has a goal that is completely contingent on the polar "needs" of the object that possesses it. This is why in physics you must always have a frame of reference: in matters of Se, you must always have something to compare Se AGAINST to make sense of it; because Se -- SLEs have said this before -- is a function of judgement by comparison.