Why use 'words' that do not exist?

The purpose of the assessment is to test phonemic knowledge.
If real words are used, there is the problem of a student recognising words, or using part of a word to make a guess at the whole word. 
Non-words (also known as pseudo-words or nonsense words) ensure that students are actively engaged in decoding all the letters.
NONWORDS, while not real, could be real words as their phonemes are not alien to English. 

Their construction follows linguistic rules - for instance, the use of syllables, real letter combinations and suffixes.

Do I need any special knowledge to use NONWORD?

NO. The manual contains full explanations of purpose, administration and analysis.  It is advisable to read this before you assess for the first time. Use the analysis pages every time you assess.

Who should be tested with NONWORD?

NONWORD is not for emergent or non-readers. It for those students (young and old) who are reading, but who have been identified as having problems with decoding words which, in turn, affects their progress.
The problem is usually caused by either not knowing certain letter combinations, or not recognizing them quickly enough for fluent reading.  NONWORD uses nonsensical combinations of letters, which can be read but have no meaning, to test phoneme knowledge.

What do I do after assessing with NONWORD?

Now that you have isolated the decoding problems, you are able to address them in a specific teaching plan, rather than aimlessly ‘plugging on’ in a general decoding drive.