Chi Sao is not a form of sparring and should never be viewed as such. Utilizing Chi Sao as sparring totally defeats its purpose. Chi Sao is used to develop contact reflexes or the ability to interpret movement through touch.
The skills one obtains from Chi Sao will greatly enhance your ability to defend yourself. Chi Sao translated means Sticky Hands. This refers to the constant contact you maintain with your partner during the Chi Sao exercise. When this absence of contact is lost the opportunity to attack maybe present.