The four corners Celtic knot traps and confuses the "evil eye" spell because the magic gets lost in the twisting and turning of the knot. The four corners knot is often called the shield knot and has been found as a symbol of protection in societies as old as Mesopotamia. Most human societies recognized their plane of existence as delineated by four earthly corners and would attribute a god to these four areas. Drawing symbols that connect the four corners increases the individual god's power to shield the human who stands behind it. The Kabbalah uses the four corner knot shape to invoke the four Archangels in a prayer called the Shema.
Prior to Christianity (about AD 450), the Celtic artwork was primarily geometric patterns. Many people have guessed as to why this, but no one really knows. The mathematical sophistication required to artistically create the commonly found patterns lead Franoise Henry to label these designs a "sacred riddle". It is assumed that the more complex a pattern the more protection one will have. The complexity of patterns can be determined by how many components are in the knot and how often the braiding of the strands changes direction. A component in a four corners Celtic knot is a continuos strand that joins itself. A component is generally recognized as a symbol for eternity, so a four corners knot with one continuous component represents protection using an eternal strand. A more complicated knot would use more than one strand which would increase the "confusion" element and thus increase the power. The individual strands that are colored differently will not be as confusing. A strand that is braided into patterns that often change direction would also increase the "confusion" element and increase the power of protection.
No one really knows what the intentions of the original artists were. Not to mention the many cultural and religious changes that have fallen on the Celtic society that has changed so many symbolic meanings. A Christian Celtic symbol could be attributed to hold one meaning, while a Nors Celtic symbol could be attributed to another meaning even though they are both the same knot pattern. The four corners Celtic knot can mean anything you want it to mean. Remember, whether it is tattoo, a necklace, a symbol on a knife, or any where else, it is a tool to focus your energy so you decide what it means and so it will be.