As Andy was fond of saying, "there's no drama". The barrel is vented at the top, and the beer served via gravity from a tap fitted at the bottom. It's the same principle as a British cask. The beer needs to turn over quickly to avoid going stale, but any popular bar dedicated to beer shouldn't have a problem. The barrels are insulated to keep the temperature down.
This method of dispense results in a perfect beer ever time, beautifully smooth with a thick, creamy head. On the few occasions we tried beer dispensed from a modern keg, the difference was startling. The beer in the glass was coarsely carbonated - often unpleasantly fizzy - due to the additional CO2 that had been absorbed when it was under pressure then forced through the lines.
Although there's only one type of carbon dioxide, forcing extraneous gas into the brew from cylinders has a negative effect on the drinking experience. Anyone that tells you lagers somehow benefit from such treatment doesn't know what they're talking about. If you try a naturally carbonated German beer alongside one served under gas pressure from a sealed keg, you'll see what I mean.