I actually had my receipt taken by the waitress on my visit but had noted the code on the receipt to do feedback, assume this was a restaurant code. However I didn't know the time that would be on the receipt so I made that up, still let me do the survey.
Quite long winded but a lot of it, you can skip, especially the scary-looking large text boxes. You are then sent a unique code by email valid any day or time for 6 months, with this very nice clear term: "3. You’re not required to purchase anything else when you dine in and redeem your voucher code, but we’d love it if you’d stay a while and dine with us."
Another very interesting point is that you can do another feedback survey based on the free dough ball visit, so it is unlimited visits, as long as you do a survey each time.
So completely free, just like the good ol' days 5+ years ago with the generic dough ball vouchers.