what happens if I turn X off?

Studying complicated structures, with multiple overlapping control planes, such as the nervous system : I find it is useful to read first 

• in terms of "what happens if I turn X off", 
• instead of "what does X do". 

They are essentially the same question - however due to cultural reasons, possibly related to the anthropology of linguistics, the positive statement "subject-has-predicate" does not generally provide details of "subject-not-has-predicate". This is quite interesting for various reasons.