The most obvious usage of knot/cord magic is for binding, the idea of incorporating knots into binding spells is a powerful one. A poppet can be sealed into a bag or pouch, their influence bound by the barrier of the bag tied with a knot. I often physically bind my poppets when I want to cast magic that will limit another person. I make a plasticine or wax figurine and wrap cord around the arms to symbolise the helplessness of the person I am binding. This can be used as aggressive magic but also for protective magic, a mother may wish to bind her child to her to ensure that child is kept safely at her side. The knots in permanent bindings can be sealed with candle wax to protect their longevity and to add a finality to the spell work.
The idea of protection can also extend to the self - many people, not just witches wear woven bracelets for protection. A cord bracelet can be woven, consecrated, endowed with protection and worn as a charm when out in the world. Herbs or flowers could be woven into the bracelet for additional protection. Some people even choose to weave the hair of their relatives or ancestors into their protective charms to draw upon the energies of those who love/loved them. This idea can extend into healing bracelets as well, with threads dipped in beneficial essential oils to assist in the healing of the wearer. (Or you could weave in a few poisons and give to an enemy...*cackle*)
Knot magic can be used to bind items to you. Some witches will bind their tools in threads infused with their own essence to personalise the relationship between themselves and their tools which are essentially an extension of their person. There is also interesting work to be done with regard to the finding of lost items where a cord is "cast out" and the knowledge of the location of the lost item "pulled" back toward the person performing the magic.
You can use cords to bind negative parts of yourself. I've done work where I have bound myself from doing things I dislike in myself and equally I have bound up parts of myself in pouches or jars as a defensive boost to my shielding. It's a shame that binding is so often shunned as being dark magic, it has so many positive uses that even those who would prefer not to use aggressive magic can happily benefit from.
After binding, the other most popular usage of knot magic seems to be the usage of the knots for storing. I like the idea of tying knots into a cord for worries and then releasing the negative energy of the worry as you overcome the problem. I've done this myself in a slightly different way by knotting each of my worries into a thread of cotton and then allowing my worries to blow away on the breeze. Other witches may burn the thread instead but the principle is the same, you store the worry inside the knot and then destroy the knot and consequently the worry.
The essence of the self can be sealed into cords for later use. Threads can be dipped in blood (or urine - don't be squicky!) to personalise them to the owner. The threads can then be used in spellwork to add additional power of the self into workings. Not only the essence of a person but the essence of the elements or the weather can be stored in cord knots. A thread dipped in natural or blessed waters may be stored to invoke the element of water in a working for example. A cord held out in a strong wind may be tied to store the power of the wind, again to be released at an appropriate moment.
Weaving is an excellent example of knot magic through the ages. Many magical practitioners would chant while weaving and weave spellwork into the fabric. Fabrics can be charmed with desirable virtues such as greater strength or compassion that will be embodied in the wearer of the garment. Some witches weave threads into a "spine" for extra courage and strength when needed. This can even be worn under the clothes as a psychological reminder of the additional strengthening energies.
Weaving doesn't have to be anything complicated. I wove a fortune wheel a couple of years ago which is a very very simple weave around four crossing pieces of wood. But my chant and my intent wove an element of luck into the charm and it now hangs in my witch room to bring me good luck. In the same way charms can be made and displayed to promoted protection or peace within the home.
Knots can be used to store your emotions. If you feel exceptional anger or despair these are excellent times to employ knot magic as the intensity of your emotion will be of use in the different workings you may choose to do. In the same way you might seal an illness into a knot and burn the thread to destroy the illness (I do this slightly differently, I store my illnesses in pebbles inside jars in case I ever want to utilise the illness for curse work, but nothing to say an illness thread wouldn't work just as well.)
Traditionally a piece of cord employed in knot magic would be three feet by three feet with the knots tied in equal sections. Again traditionally, nine knots would be tied though the colour of the cord will differ depending on the individual witch and also - if a witch chooses to use colour correspondences - she may use a colour of cord appropriate to what she is trying to achieve.
Knot magic plays a part in many everyday and special activities of the witch. Some witches choose to denote their sacred space with a cord long enough to mark out the circle. This acts as a physical marker of the magical space created by their mind and can be a useful tool in group work. Handfasting, the "Pagan marriage" is a ritual where lovers are symbolically tied together to celebrate their union. The knots seal the love between the two people.
In many a modern home, magical or not, there often hangs a dream catcher, an intricate weaving of threads designed to trap the good dreams and allow the nightmares to pass through. A dream catcher itself is essentially just another usage of knot magic.
My own take on knot magic is heading in quite a unique direction. I am currently playing with the idea of anchoring myself to the magical places where I find spiritual solace. For me it is Scarborough on the East coast of England and on my next visit I intend to find a quiet spot and anchor some knot ties to "pull" me back to the place when I feel I have been away too long and I need to be spiritually replenished. The other great interest of mine of course is my tarantulas and I look to both utilise their webbing in the work that I do and to create some webbing of my own as a piece of cohesive magic to link together the threads of my life in a positive and unified whole. So as you see, the possibilities really are as long as a piece of string.....