Most types of forbearance are not automatic you need to submit a request to your student loan servicer, often using a form. For some types of forbearance, you must provide your student loan servicer with documentation to show that you meet the eligibility requirements for the forbearance you are requesting.
Also, for Parent PLUS borrower deferment. If I quit paying my student loan, what will happen to my requested deferment?
if you stop paying your loan before you've been notified that your request for deferment has been granted then your deferment will not be approved and your loan will become delinquent and you may go into default.
and what if I'm eligible for the deferment and rolled for at least half-time?