[majormajor]

The second levels are pretty similar - push harder than you would normally, pushing "through" your usual action, get haptic feedback, something else happens. One difference that helps discoverability, though, is that 3D touch for stuff like app shortcuts and "live photo" replays has UI-based feedback to tell you there's more if you push harder, whereas the second level push on the MacBook trackpad feels less rewarding to experiment with (or maybe that's just a function of it being used far less in OS X than in iOS).