Exam Certified Platform Developer II topic 1 question 191 discussion

You can use the setRedirect attribute on a pageReference to control whether a postback or get request is executed. If setRedirect is set to true, a get request is executed. Setting it to false does not ignore the restriction that a postback request will be executed if and only if the target uses the same controller and a proper subset of extensions. If setRedirect is set to false, and the target does not meet those requirements, a get request will be made.

This type of redirect performs an HTTP GET request, and flushes the view state, which uses POST. If set to false, the redirect is a server-side forward that preserves the view state if and only if the target page uses the same controller and contains the proper subset of extensions used by the source page. Note that if the URL of the PageReference object is set to a website outside of the salesforce.com domain, or to a page with a different controller or controller extension, the redirect always occurs, regardless of whether the redirect attribute is set to true or false.