Final answer:
A keg tap or keg coupler is a device that creates an opening in the keg for beer to flow out, and it functions by attaching to the keg valve and using CO2 pressure or a hand pump.
Step-by-step explanation:
The mechanism that allows beer to flow out of a keg is known as a keg tap or keg coupler. This device is responsible for tapping into the keg and creating an opening through which beer can flow. It usually operates by attaching to the keg valve and then allowing CO2 pressure or a hand pump to push the beer out. The keg tap has several components, including a tapping head, which connects to the keg, a coupler, which attaches to the tap and opens the keg valve, and hoses that transport the beer and CO2.
There are different types of keg taps depending on the keg system or the type of beer, but the most critical part of the mechanism is the proper handling and connection to ensure a smooth flow of beer and no leaks.