Contagious Magic, or the Law of Contact relies upon the belief that two things once connected still are in some fassion after being physically separated, such that an action on one produces a similar effect on both. It is a kind of sympathetic where like affects like, but rather than being immitative, it involves two things once in contact said to be energetically linked.
In practice, this means somebody can work magic on another person by having some part of their body such as hair, teeth, or nail clippings. Harm done to the pieces happen to the person in this kind of magic. Also, the fragments could be used in a healing or protective charm. This is why torn pieces of the target's clothing and their hair are used to make a poppet of them.
Footprints and paths of travel are said to keep a non physical connection to people. Nails driven are into a footprint so the victim will suffer an injury. Urinating or putting a mix of hot pepper, crushed insects, and other unpleasant things in the footprint are done to keep a person away from an area. Also, the earth from the footprint can be collected to be used in other magical workings.
In addition to direct magic, this theory also applies to spirit work. Practitioners may use parts of a dead animal in ritual because of their connection to that animal's spirit. In necromancy, mediums may use a personal item that belonged to the spirit they wish to contact. People say the "psychometric vibrations" of the item help the medium tune in to the energy of the spirit. Some believe that even photos capture the energy of a person and can be used in this way.
For discussion:
What kinds of contagious magic do you believe work if any?
When I was learning, it was always termed as the Law of Contagion. Also, when you say direct magic, what is your context? Different people tend to have different understandings of what terms are and it would be helpful to understand the context you intended.
Folk practice and spirit work are common ways to put the "Law" of Contagion into use as well as they are a part of my practice (more in spirit work and in ritual).
I've used different sorts, both direct and indirect contagious magic. I've created things that have represented people (namely sigils and seals), I have used hair from someone else before, I have done a handful of other things. However I don't feel it necessarily works with all things. There are things you touch once, then after that they are touched many, many times over. I don't think such things would necessarily hold or contain some portion of your energy or essence unless you directly charged them during that one encounter.
It's not just if someone has touched an item, it's also if their essence has touched it which is a bit different. By essence I mean the essential quality of their energy; it's a deeper thing than most energy used by people because people tend to use their outer/external energy or energy that is external to them. It's something I should really make a proper post on, but this essence is the energy that is within us, it represent us to our outer energy and the outer world. If someone's outer energy touches something, it is temporarily connected. Eventually it will fade with time. However our essence will remain for a long, long time; depending on the contact it can be permanent.
