Sigil magick is primarily understood from 3 different theories of understanding, these theories being the way of understanding how sigils are able to manifest. Practitioners will usually work with one of these theories, and will understand sigil magick from that point of view. From what I understand all of these theories are legitimate, and should not be looked at separately, but much rather together in order to have a higher understanding of the ins-and-outs of sigil manifestation. These theories I like to refer to as energy theory, consciousness theory, and psychological theory.
Energy theory is the understanding that sigils work through the process of gathering up spiritual energy that is intended for a specific purpose, and when it is cast out that energy will go forth into the universe to manifest that intention. This is a theory that primarily works off the understanding of spiritual energy, and raising that energy in order to do a certain task.
Consciousness theory is the understanding that sigils work through implanting sigils in the subconscious, so that they will be able to interact with the universal conscious in order to manifest a desired outcome. This theory works primarily off the understanding of universal consciousness, and how we are all connected to it, and because of that are able to influence it in order to make our desires a reality.
Psychological theory is the understanding that sigils work through the act of allowing you to focus on a specific construct, or situation, through the representation of the sigil. This is said to allow your subconscious mind to pull you in the direction of what you desire, by it suggesting to your normal waking consciousness certain decisions that will help you achieve your goal.
There are critiques, and criticisms to all of these theories but all of them support a very good understanding of the essence of sigil magick, and are all important to get acquainted with.