May I ask a few questions : - what the "Back" triangle is for when there is a back arrow on the top left of the app, doest it perform the same action ? - As the "circle" is home, I guess there is no way to hide this taskbar as you could not go back to home screen, right ?
For example if you have a list of items and you click on one, then on that item you have related items and you click on one, the up button will take you to list where as the back button will take you to the previous item.
And you're right, you can not hide the taskbar.
You can find more information here: http://developer.android.com/design/patterns/navigation.html
I followed the link you provided and it seems the previous android version had different taskbar icons, which I found a bit easier to understand.
Still, Android and iOS do not help much with discoverability in my opinion, and the newest the version, the worst it is. What's next, heptagon for sending email and decagon for contacts ? I feel so old :(