Audi makes a special tool available which is used to lock the crankshaft in place while loosening and retorqueing the harmonic balancer bolt into the end of the crankshaft. This is important since a very high torque is used, and there is no convenient or sensible way to stop the motor from turing when doing this. Since the tool costs about $75 and is not always easy to find, people also come up with their own solutions based on how it works.
Here is what the "real" 2084 tool looks like:
And here are a few homemade solutions (these pictures were sent to me as a result of someone's mailing list query about the subject, and I do not know where they originally came from):
Here someone is using a typical pry bar to jam the nub inside the harmonic balancer from turning, with the pry bar catching against the snubber front mount thing just like the real tool:
Here are some photos of a homemade substitute:
Cut a piece of 2 inch square stock as shown:
Set up for use with a pipe wrench as the locking "tab":