The theory of Anti-magic is a simple one, yet the execution of such an ability is incredibly complex. Only the most powerful of magic users who understand the usage of magic at a fundamental level can possibly comprehend the precise nature of the use of anti magic. The basis of the theory follows below:

When a magic user ‘senses’ magic, it is because when a spellcaster casts magic, they give off a unique wavelength. Each wavelength is unique to each person and their magic, so if you were fine tuned enough, you would be able to tell the difference between spellcasters based purely on their magic. Therefore, an anti-magic would have to examine the spellcaster's wavelength and create a 'null' wavelength, thus unraveling anything the caster tries to do. Though there are few major weaknesses to Anti-magic: 

1) It will take the device or user a relatively large amount of time (say a few minutes) to figure out the exact wavelength of a person’s magic. 

2) a single person intending to cast anti-magic will not be able to null the magic of one more powerful than themselves. This weakness can be overcome by multiple people combining their ‘null magic’ wavelength into one more powerful stream. 

3) to efficiently detect a magical wavelength, a very specifically crafted artifact must be constructed in order to do so. In most cases, this artifact must have physical contact with the enemy caster.