The Function of the Mazal:[1]
The Mazal is the level of Ayin which is completely nullified to G-d and is not considered a separate existence from Him. The Mazal serves as the channel to drip G-dliness into the soul below and effect it with feelings of repentance, as the ray of the soul which descends into the body is not completely separate from its root and source above and hence remains connected to its Mazal to receive revelation from it. This, however, is with exception to one who transgressed a sin that is liable for excision, in which case the connection of the ray of the soul below with its Mazal has become cut off and disrupted and is thus unable to receive the messages from above until he repents.
[1] Likkutei Torah p. 71b