To enable automatic promotions, click settings > basket > check "Check Promotions On Checkout".
Once this is done you can create a new basket and simply click "checkout" to automatically apply discounts which have been set as "automatic" in admin.
You can set up a promotion (see set up a promotion) which can be applied using the coupon button at the top of the basket.
Tap select from list to display your promotions. (You must have a working internet connection for this bit)
Promotions can be set up to apply any discount from 1-100% to a product, category or sub category. In this case the cake is the only valid product.
Note: You cannot alter the basket when a promotion is applied. To remove a product once applied, tap on it in the basket and select 'Delete'.
In this example a 10% discount on all products is applied: