A touch of my human finger, Pal. Nothing more. In the picture you can actually see the finger tip shaped crescent where the crack started.
Moreover, I don't have any reason waste my time coming on this forum to lie. I'm mostly resigned to the fact that German Apple is not going to fix this for me--but I'm surprised enough that it happened to say something about it, especially when I see others are having similar problems.
I never expected the track pad to break in-hand, particularly after receiving pre-purchase assurances from misinformed folks like this excited little person here that the glass is indestructible, chiseled by ninjas from unicorn horns, etc. I suppose they are experts on tempering glass and this problem is all in my head. Or maybe Apple just tried to design something too cool too fast.
If anyone else has cracked track pads--due to normal or abnormal use--please post (accurate) info and pictures. I think this is an issue worth knowing about.
To be honest with you, if you push hard enough with your finger, I bet you CAN crack it with finger force.