Basically you pay in full at checkout, send in your trade-in device once you received their return package. If your trade-in gets approved, you'll get refunded the difference back to your card. If you return the new iPhone before it gets approved, they'll send you a Apple gift card instead.
I've done this before, got approved with full credit, and was refunded the difference back to my debit card. Tempted to trade in my 12pm for the 13pm.
Example below, total I'll pay if I get full $790 credit is $513.91