The service works with the tastecard app (for Android and iOS) which you can use to search participating restaurants and to book tables (which you must do in advance). Simply present your card when you pay your bill after your meal.
tastecard also gets you 2 meals for the price of one. This is valid for two people only, where the cheaper dish of each course ordered is free. Read more about how tastecard works.
You can also opt for a 60 trial of tastecard for £1 which gets you all the features in the fully paid version.