AppleInsider has the best rundown of Apple's new installment plan option for the iPhone 6s/6s Plus:
While it lacks the fanfare of new hardware or software, one of the biggest announcements from Apple this week was its competitive iPhone Upgrade Program. It's actually an interest-free way to pay off your new iPhone and AppleCare+ warranty over a two-year period, with the added option to upgrade your iPhone to the latest model every year.
Unlike installment plans offered by some wireless carriers, Apple's iPhone Upgrade Program features a completely unlocked iPhone. In addition, the program is also interest free, so the loan comes at no cost to you.
In other words, the price you pay over two years is identical to what you would pay upfront for a new, unlocked iPhone 6s and AppleCare+ two-year warranty.
The reason this no-interest loan program works for Apple is that they get your used iPhone back if/when you upgrade (you don't have to, but I'd imagine most will). The secondary market for iPhones is huge, especially in China and India, and Apple will claim a big piece of it by controlling the flow of one-year-old iPhones.