The SP plan focuses on eating low energy denisty foods.
To follow a true SP day you need to avoid pasta, rice, potatoes, and any syn free foods that don't have an S or P next to them in your SW Food Optimising book (or on the website!)
You still get your 5-15 syns a day, and they can be used on whatever you like, as normal. For example, you can't have any syn free yogurts, but you can use your syns to have a yogurt if you wish.
You also get 2 hex b choices to use.