It's a method of more uniquely identifying what an interchange (co-aspect) means in the context of a specific person's natal chart - things we miss or don't think of when applying interchange interpretations more generically. It relies on principles with which everyone probably agrees, builds on what we already understand of aspects, and shows us - at a glance, intuitively! - where the specific interchange falls in terms of impact in a specific person's life and psyche.
Consider the following: When we meet and begin to interact with a person, we arrive having already our natal horoscope and its aspects. If one of your planets aspects one of my planets that natally has few (or wide) aspects, then your planet will have a much more powerful impact than my own planets. For example, my Sun is essentially unaspected: If someone's planet aspects my Sun, they have a stronger impact on it than anything in my chart has except its Virgo sign placement.
Similarly - same principle - any planet you have aspecting my planet comes into a psyche / person / chart system that already has various configurations on my planet. My mix of aspects is much more complicated than simply the planet interchange. If you brought Jupiter to my Venus and I already had Venus conjunct Saturn, I would react differently than someone who, say, had a Venus-Moon aspect instead.
In some ways, this "how do you take into consideration each person's natal chart when judging synastry," or "how do you really consider what they are bringing to the relationship," is the long sought Golden Fleece of synastry. Everyone says you should do it - and nobody has proposed a way of doing it (other than the Tropical-heavy idea of house rulerships in which your Uranus on my Sun means it affects my Sun-ruled 12th house and your Uranus-ruled 11th house - which is rubbish). Repeat: I don't know of anyone ever providing an actual method of discerning what each person brings to the mix of energies other than the reasonable global overview of read the entire chart and get an idea what kind of person this is and what they want in a relationship. (And this approach, as theoretically sound as it is, remains, at best, unsatisfying in real-world practice.)
But there's an easy way to do - an absurdly easy way!
Simply collate the interchange into the table of "planetary state" TMSA has - i.e., add the interchange into a natal aspect sort by relative strength - and the astrologer will intuitively, quickly, and accurately know what to do with it!
Consider: My wife's Venus opposes my Mercury 2°08' and her Mercury opposes my Venus 3°22'. At first look, the aspects are straightforward and accurate: Smooth, flowing, fluid communication has come easy to us since we first met. When we were building our relationship, we'd spend hours (often until dawn) talking together non-stop.
But it's more complicated than that. Even ignoring for a moment that her Mercury and Venus each have their own very strong aspects, these planets are aspecting my own Venus and Mercury with their own aspects. For example, my Mercury has one natal aspect, a 2°24' conjunction with Saturn. The more deeply I settled into connection with my wife, the more my Mercury was also in 2°08' opposition to Venus (in this case, her Venus). This means that my Mercury changed from one aspected only by Saturn to one aspected by Saturn and (even closer) Venus.
Just how much weight do I give this "new" aspect? (This is always an important question, right?) The answer is that, if I collate the interchange into my list of things affecting my Mercury, it will become quickly obvious to me - I don't need new rules - it will simply be evident to me. I will be able to see, usually at a glance, how strongly that one aspect affects my Mercury, and almost as quickly see how important it seems in my overall psyche. It's a strong measurement of exactly how I "become a different person" while in relationship with her.
Here is a concrete example. First, I will create a "state" report similar to what I think TMSA will soon have. This is just my natal chart:
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Mo Aqu FG | sx Ma 1°31' tr Ve 4°29' tr Ur 5°56' tr Ju 6°13'
Su Vir | sq Ma 6°28' co Ne 8°53'
Me Lib | co Sa 2°24'
Ve Sco- | co Pl 0°13' tr Ur 1°27' tr Ju 1°44' sx Ma 2°57' tr Mo 4°29'
Ma Sag | sq Ne 0°07' sx Mo 1°31' sx Ve 2°57' op Ur 4°45' op Ju 4°41' sq Su 6°28'
Ju Can+ | co Ur 0°17' tr Ve 1°44' sq Ne 2°16' op Ma 4°41' tr Mo 6°13'
Sa Lib+ | co Me 2°24'
Ur Can | co Ju 0°17' tr Ve 1°27' sq Ne 2°00' op Ma 4°45' tr Mo 5°56'
Ne Lib | sq Ma 0°07' sx Pl 0°46' sq Ur 2°00' sq Ju 2°16' sq Ma 0°07' co Su 8°53'
Pl Leo | co Ve 0°13' sx Ne 0°46'
Now, I'm going to modify it only by adding the aspects (the ones I consider valid in synastry) that my wife's planets make to my chart. This will tell you the "different person" I am to the extent I'm in relationship with her, and also show how her aspects to my chart fit into the mix and balance of how my chart is on its own.
In the following table, I have not calculated mundane aspects (of her planet to mine in the framework of my chart), but perhaps I should. In a case where I have planet A aspect planet B, and she also has B aspect my A, I have gone with the smaller orb (e.g., my 0°46' Neptune-Pluto sextile wins over her 1°00' semi-square of Pluto to my Neptune).
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Mo Aqu FG | sx Ma 1°31' tr Ve 4°29' tr Ur 5°56' tr Ju 6°13'
Su Vir | oc Ur 0°21' op Ju 4°30' sq Ma 6°28' co Ne 8°53'
Me Lib | co Ne 2°04' op Ve 2°07' co Sa 2°24'
Ve Sco- | co Pl 0°13' oc Ju 1°05' tr Ur 1°27' sq Ma 1°58' sq Sa 3°00' co Me 4°22' tr Mo 4°29'
Ma Sag | oc Su 0°06' sq Ne 0°07' oc Pl 1°25' sx Mo 1°31' sx Ve 2°57' op Ur 4°45' op Ju 4°41'
Ju Can+ | co Ur 0°17' tr Ve 1°44' sq Ne 2°16' op Ma 4°41' tr Mo 6°13'
Sa Lib+ | co Me 2°24' co Ne 4°29' op Ve 4°31'
Ur Can | co Ju 0°17' tr Ve 1°27' sq Ne 2°00' op Ma 4°45' tr Mo 5°56'
Ne Lib | sq Ma 0°07' oc Mo 0°21' sx Pl 0°46' sq Ur 2°00' sq Ju 2°16' sq Ma 0°07' co Su 8°53'
Pl Leo | co Ve 0°13' sx Ne 0°46' co Ma 1°45' oc Ju 0°52' sq Sa 3°14' sq Me 4°35' co Ur 5°01'
What kind of difference does this make? For one huge difference, look at my Sun in the two charts and think how you would interpret the natal Sun of those "two people." Or notice how her Saturn square my Venus, though not a very wide orb, is only the fifth-strongest aspect to my Venus and takes a proportionately minor role, while her wider Jupiter opposition to my Sun (because of where it falls in the mix of my chart) is dramatically more important). Or how her Jupiter to my Venus makes my natal Venus-Jupiter stronger (and, by nature of the aspect, more dynamic). There is much to see!
For another example, here is my "state" report with Mikestar13's aspects to my planets added in. Look at how it shifts attention on my Moon. Notice that his Mars to my Neptune doesn't even appear - seems to have no effect - because my own Mars-Neptune is already much stronger. Similarly, his Mars to my Moon pales before my closer natal Moon-Mars, but his Moon to my Jupiter-Uranus is much stronger than my natal Moon-Jupiter and Moon-Uranus. There's more to see:
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Mo Aqu FG | sx Ma 1°31' op Ju 3°22' tr Ve 4°29' tr Ur 5°56'
Su Vir | sq Ma 6°28' co Ne 8°53'
Me Lib | oc Ju 1°30' co Sa 2°24'
Ve Sco- | co Pl 0°13' oc Su 0°39' tr Ur 1°27' tr Ju 1°44' sx Ma 2°57' tr Mo 4°29'
Ma Sag | sq Ne 0°07' sq Me 0°46' sx Mo 1°31' sx Ve 2°57' op Ur 4°45' op Ju 4°41' sq Su 6°28'
Ju Can+ | co Ur 0°17' sq Mo 1°19' oc Sa 1°29' tr Ve 1°44' sq Ne 2°16' sq Me 3°56' op Ma 4°41'
Sa Lib+ | oc Ju 0°54' co Me 2°24'
Ur Can | co Ju 0°17' sq Mo 1°02' tr Ve 1°27' oc Sa 1°46' sq Ne 2°00' sq Me 3°39' op Ma 4°45'
Ne Lib | sq Ma 0°07' sx Pl 0°46' op Mo 0°58' sq Me 1°39' sq Ur 2°00' sq Ju 2°16' sq Ma 0°07' co Su 8°53'
Pl Leo | co Ve 0°13' oc Su 0°26' sx Ne 0°46'