____roof of mouth_____\n\n ______________\n /\n/
Now say the German word "boot". Your tongue will be shaped like this (side view):
____roof of mouth_____\n\n ______\n / \\________\n/
The tongue doesn't actually touch the roof of the mouth, but the arch is higher and more forward, and the tip of the tongue depresses to assist this.
In addition, the German final 't' is often clipped shorter than the American, which is nearly aspirated (assuming it's not followed by another word, as in this example).