What occurs to the hard spile when beer is being served from a cask?

When beer is served from a cask, the hard spile is typically removed to allow air to replace the beer that has been drawn out. This process is essential for maintaining the proper flow of beer from the cask. As beer is dispensed, a vacuum can form which necessitates the introduction of air to prevent a buildup of negative pressure. The removal of the hard spile facilitates this, ensuring that the cask can continuously provide beer without interruption. The introduction of air helps balance the pressure inside the cask, allowing for a smooth and consistent pour.

Other options suggest processes that are not accurate in the context of serving from a cask. For instance, replacing the hard spile with a soft spile or leaving it in place would not adequately address the pressure dynamics required during serving. Similarly, using it to add carbonation is not a function performed by the spile. Instead, the hard spile's removal is a critical step in maintaining flow and pressure balance as beer is served from a cask.