Magical links follow Sir James Frasier's two primary laws of magick, generally refered to as the Law of Similarity and the Law of Contagion. In "The Golden Bough" Frasier writes:
"If we analyze the principles of thought on which magic is based, they will probably be found to resolve themselves into two: that like produces like, or that the effect resembles its cause; and, second, that things which have once been in contact with each other continue to act on each other at a distance after the physical contact has been severed."
The first principle, the Law of Similarity, states that when any two objects share a particular property that property can allow one to influence the other. The more properties the two objects have in common, the stronger the link. The stongest similarity link is going to be a current image of the target, such as a recent photograph. The second principle, the Law of Contagion, states that once any two objects have come into contact one can be used to influence the other. The closer the contact, the stronger the link. The strongest contagion link is going to be created by something that not only came into contact with the target but which was once part of it, such as hair, blood, fingernail clippings, and so forth.
A third law of magick has been proposed by some practitioners, generally refered to as the Law of Opposites. This law states that when two objects are of opposite natures one can be used to influence the other. As far as I can tell from my own work, though, this law is essentially useless in the context of practical magick. I hypothesize that The reason for this is that the meaning of "opposite" is subjective. Is red the opposite of green? Male the opposite of female? Dog the opposite of cat? Salt the opposite of pepper? When you start trying to break things down like this it gets silly pretty fast. And if you're sloppy about your definitions you wind up with a tautology - similar things influence each other but so do dissimilar things. That is, everything influences everything else. From the standpoint of ritual design this is not helpful.
The Law of Opposites does have a particular application in mystical spirituality. For the development of spiritual realization it is important to understand that when you define X you automatically define Not-X as well. For Good to exist as a distinct thing Evil must exist as well, in that Evil can also be defined as Not-Good. Furthermore, the process of enlightenment involves, among other things, the evolution of a perspective that can accomodate both as complementary rather than exclusive. When talking about properties of the physical world rather than components of consciousness, though, this utility is profoundly limited. It is telling that the last group clinging to the Law of Opposites as applying to the physical world, practitioners of homeopathic medicine, have largely abandoned it in favor of the more similarity-based "water memory" model over the course of the last two decades.
What magical links conduct is probability shifts. When you cast a spell on an object linked to a target, any influence cast on the object will also surround the target. The strength of the transmitted shift will be directly proportional to the strength of the link. This may be modeled as a simple percentage, in which we for example would define a link that transmits 80% of a shift a link of.8, a link that transmits 50% of a shift a.5, and so forth. This is essentially how the L (Link) variable is used in both my operant equation and Peter Carroll's original equation on which mine is based. Direct contact is always a 100% link, or 1.0, whether you are casting a spell on yourself or on someone or something that is physically present in the temple. What this means is that most of the time casting over a link is going to be at least somewhat less effective than casting directly.
A trick that Hoodoo practitioners commonly use to get around this problem is to enchant powders, oils, or other substances and place them strategically so that they will come into direct contact with the target. This results in the creation of a 1.0 link due to direct contact even though the spell itself was performed earlier in a different location. The ceremonial schools raely teach or work with this method but it is easy enough to adapt using a resource like "Liber 777", which lists various herbs, incenses, and materials associated with particular powers. Any material appropriate to a particular power can be used to contain a shift related to that power, which provides the ceremonialist with many different options. As long as the target comes into contact with the enchanted material the spell's probability shift will be transferred optimally.
Mike's first question was in regard to the use of names as magical links. Essentially a name can be used in two ways, as a weak similarity link and as an identifier. You can cast a spell giving only the target's name as a link, but this is not a particularly effective method. You generally want to augment it with a better similarity link like a photo and/or a better contagion link like hair clippings. As an identifier, though, names are much more useful. For example, let's say that you enchant a powder to dump across your target's doorway and they live in an apartment building. You don't necessarily want everyone walking in and out of the building to be affected by the spell, just the individual you have targeted. So in your enchantment your statement of intent would include the target's name. That way the shift will only transfer to the named individual, not to anyone who touches the powder. This also renders the powder safe for you, the magician, to touch without risking exposure to the spell's probability shift.
Mike's next questions were related to whether or not links could be used to target "energy work" based magick, such as Qigong healing. In my experience the answer is yes, but there are some limitations involved. Chinese researchers have proposed that Qigong's healing effects may at least in part be produced by infrasonic waves emitted from the hands of Qigong masters, and such waves cannot be transmitted by a magical link. However, energy work also seems to include a probability-shift based component and that can be transmitted just like any other magical effect. I will say that I have been able to produce distance-healing effects that incorporate Qigong, but direct contact has always worked better. I suspect this is due to the absence of infrasonic waves when performing a distance-healing, not necessarily just because the shift degrades when passing over the link. If that's the case there's going to be an upper limit to this sort of effect and direct contact is always going to produce the best results.
The best way to build a strong link is to exploit the two basic rules to the greatest possible degree. In other words, you should have a current image that if at all possible is augmented by something that has come into contact with or come from the target. In theory, one of the best possible links you could have to a target is a Polaroid instant photo, seeing as the photons from the target will have made direct contact with the picture, representing both a similarity and a contagion link integrated together. This may mean that magicians could be a new market for Polaroid cameras, which are unfortunately being replaced by digital cameras that only create similarity links.
The image accompanying this article shows how these principles are applied in radionics - that is, magick performed by an electronic device. The TARGET LINK is what I'm talking about in this article. The TREND is the term that radionics practitioners use for the probability shift. The TREND LINK doesn't really have a correllate in ceremonial magick, but in radionics it represents the connection to the probability shift that you are creating, which is envisioned as being related to a particular energy frequency. The LIFE FORCE FIELD is what I refer to as the general field of consciousness, which may be represented as consisting of information, energy, or both depending upon one's model of what consciousness is made up of.
UPDATE: It seems I spoke too soon about there being no real equivalent to the radionics "trend link" in ritual magick. As I replied to Frater Barrabbas in the comments, a device like a planetary tablet or some other symbolic representation of a particular magical power or vision would seem to fit the bill. So for a Jupiter ritual, my usual go-to example, the magical qualities of Jupiter would correspond to the Trend and a device bearing the symbols, characters, and names of power associated with that planet would correspond to the Trend Link.
Suggested ebooks:
Jarl Fossum - Seth In The Magical TextsPeter De Abano - Heptameron Or Magical Elements
Aubrey Bell - The Magic Of Spain