What does the inner wooden aug, or keystone, do when tapping a cask?

The inner wooden aug, or keystone, serves the important function of guiding the placement of the tap when tapping a cask. It fits into a specially shaped opening at the top of the cask, ensuring that the tap is aligned correctly and made with a proper seal. This alignment is crucial for preventing leaks and allowing for a clean flow of beer.

In the context of a cask, the keystone helps maintain the integrity of the cask's structure during tapping, which is vital for both operational efficiency and maintaining the beer's quality. While the other functions of flavor infusion, pressure release, and contamination prevention are related to the cask's overall operation and integrity, they are not the primary purpose of the keystone itself.