That would be the "Send free download link/code" item in your Seller Admin, and that will only work for digital products (ones which have a File Download, Send Codes, or a product-based Redirection setting enabled).
For testing the cart, we recommend using our Free Checkout option by either temporarily setting your product price to 0.00, or creating a secret Cart Discount Code for 100% off the Item Total, so you can enter that code in your test cart to activate the Free Checkout option for your test order.