The new cameras on the iPhone 6S are nice, but the biggest technical innovation Apple crammed into the thing is the screen. Actually, it's just underneath the screen: the sensors that enable 3D Touch. If you haven't been following, 3D Touch is Apple's branding for the new pressure sensitivity feature on the iPhone 6S and 6S Plus. But where we've seen pressure sensitivity mostly used on drawing apps before, Apple has built its software to support it directly in the UI. Press a little harder and things start happening. It's like Force Touch on the Apple Watch and the newer MacBooks, but now it's on the most popular device Apple sells.
It sounds like a gimmick, honestly, but the truth is it's not. When you use it, you quickly realize that there's a logic behind what 3D Touch does in Apple's apps. (As of this writing, it's only in Apple's apps — but we expect that to change really soon.) So what can 3D Touch do and what's the logic behind it? Well!
3D Touch is best for getting a little preview of the next thing
The main thing to keep in mind is that Apple's own apps mostly use 3D Touch for getting a little preview of the next thing you want to do. So if you're on the home screen, hard press on an icon to get a few shortcuts directly into a section of the app — like taking a selfie from the camera or calling one of your favorite contacts.
This "preview what's next" idea really comes into its own inside Apple's apps: Mail, Messages, Safari, etc. In those apps, Apple's got a couple more marketing terms to learn: "peek" and "pop." The basic idea is this, you can hard-press on an email or link or address or anything else the iPhone understands to get a "peek" window. It's a small preview of the thing being linked, whether it's a web page or the email you want to peek at from your inbox. It's not immediately intuitive, but it's incredibly useful, and once you start using it you'll really get into the habit of just taking a quick look at a thing before getting back to your conversation.
Once that peek window is open, you have a few options. You can just lift your finger to close the window, of course. But if you like what you see, you can "pop" it; you press just a little harder to open up that preview window full screen in its app. (And since iOS 9 has these new, handy, breadcrumb-style back buttons in the upper left, it's easy to get back, too.)
There are other options in the peek view, though. Slide the window around and options will appear. Slide up on a Safari preview, and you'll see options to open it, copy the link, or just add it to your reading list for later. In email, you can slide up for a bunch of options, too. Or you can slide it left and right to mark it read or archive the email (those options mirror your default swipe options on the main inbox, which is clever).
Apple keeps 3D Touch consistent and intuitive
The consistent use of peeking and popping is my favorite thing about 3D Touch. It could have been like the Apple Watch, where a Force Touch is basically a random input method for random options, but instead it's a totally predictable and totally useful feature on the iPhone. I can't wait for developers to begin implementing it — especially since I don't actually use many of Apple's core apps like Mail, Calendar, and Maps. And they're free to try other things with 3D Touch too, but Apple is making the defaults for peek and pop available for everybody to help maintain some consistency.
Apple does have some other random things you can do with 3D Touch. If you use some pressure when you swipe in from the left of the screen, it'll jump you into the multitasking view. I don't know if that's really any better than just double-clicking the home button, but it's there if you want it.
The keyboard has a neat 3D Touch feature, too: press hard on it and the keyboard will gray-out and turn into a trackpad, so you can quickly and accurately place the cursor where you want it.
Last but not least — as far as Apple is concerned, anyway — are Live Photos. They're Apple's way of trying to bring images to life by giving them some movement. When you take a photo, the iPhone 6S can record about a second and a half of video before and after your snap. Then, you can 3D Touch any photo to view that little clip. Same thing applies to your lock screen — press it hard and you'll see your wallpaper move around. It's fun, but not strictly the kind of thing that really requires fancy pressure-sensitive screen technology.
If you look closely when you 3D Touch an icon, you'll see the screen blur in proportion to how hard you're pressing. As Nilay pointed out in our main review, it's the sort of thing that just shows off how tightly integrated the hardware and the software are here. But the most impressive thing isn't the whiz-bang spec, it's how thoughtfully Apple has implemented it. I fully expected to be confused by 3D Touch without practicing with it and figuring out how and where it works. Instead, I got it almost immediately. And if you spring for a new iPhone, I suspect you will too.
Read next: Our iPhone 6S review.