After a diner sends feedback, the thank-you page offers them a button to leave a Google review. By default it is shown to every diner, whatever the rating. That is what we recommend.
Sending only the happy diners to Google and the unhappy ones to a private form is called review gating. Google's review policy prohibits it, and the US Federal Trade Commission has warned that it can be deceptive. We never switch it on for you.
The option to show the Google button only from a minimum rating exists, because it is your restaurant and your decision. Turning it on shows this warning, and it cannot be saved until you confirm that you understand the risk. It is never on by default.