DuQana's discounts are "Buy X Get Y Free" promotions set up on individual products. There's no separate percentage-off or coupon-code discount, and cashiers never type in a discount at checkout.
Setting up a promotion
On a product's form (any type except Bundle or Variants), switch on "Enable promotions / discounts" and add one or more rows: a label, a Buy Qty, and a Free Qty.
How it applies at checkout
There's nothing for staff to turn on: the moment a customer buys enough of a product to qualify, DuQana applies the best available promotion for that product automatically and reduces the total. If a product has more than one promotion, only the one that saves the customer the most is used; they don't stack.
On the receipt
When a promotion applies, the receipt shows the promotion label and the amount saved, so the customer can see why the total is lower than item price × quantity.
Not the same as Variable Price
Variable Price products let staff type a different price within a min/max range at checkout (see "Understanding Product Types"): that's a per-sale price choice. Promotions are a fixed, automatic rule tied to quantity bought, and apply the same way every time.