You basically have to push hard on the edge of the glass. Pretty much on the curve.
Not really. In fact, too close to the edge makes it harder. There is a very generous zone to register the multitasking view.
To add on top of the excitement in this thread: Light press then swipe to switch between the last app opened. Hard press to actually go to recent apps list.
Edit: To clarify. To activate multitasking view aka app switch view you just have to make sure a part of your thumb is off screen. Imagine a drawing app. If you just press with your thumb, the app will draw, ideally, a perfect circle. To trigger the app switch view you just have to make sure that the phone doesn't register a perfect circle. So big thumb or little, just make sure flesh is off screen.
Try it yourself. Touch the very edge and then press hard. Move your thumb further into the phone area but make sure some part of your thumb is still in contact with the edge of the screen. The trigger will works.
Also, to make sure 3D Touch is receptive, touch your screen first before pressing. I had the habit of just immediately hard pressing things and getting wonky results. You gotta give the phone a split second to warm up.