That just looks like the material of the trackpad bending down under pressure. If you press down the uppermost part of a non-force touch trackpad you'll see that it bends down very similarly as well, even though it shouldn't move at all.
Which brings me to a related problem that hasn't been addressed so far: Take your iPhone and apply a force to the screen. Maybe it's not a problem with more recent iPhones, but on my iPhone 5 this causes the screen to do change color in weird ways at some point. That's just the screen bending, causing the liquid crystals of the screen to change properties. I guess Apple needs to address this as well.
Happens to me on my iPad, too.
If I press too hard, the area around the finger goes haywire—the colours go bonkers.