He's spunding, which means letting the fermentation finish in the keg. Fermentation of course produces CO2, and by allowing that CO2 to stay in the keg, it self-carbonates the beer, much like how bottle conditioning carbonates beer.
The spunding valve allows him to maintain a specific pressure during this time. It releases pressure when it gets higher than he wants.
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He has it set at 21psi, which normally would be too high for serving beer. But the beer must be kept at ferm temps, which would be, for example, in the mid 60-degree range.
Let's suppose he's got it at 63 degrees. According to the chart below, 21 psi at 63 degrees will give him 2.16 volumes of CO2 in his beer. When it's done fermenting, if he lowers the temp of the finished beer to, say, 40 degrees, the beer, being colder, will absorb CO2 more readily. That will drop the pressure to between 8 and 9 psi.
You can see this in the chart by looking across the 40-degree line until you find 2.16 volumes of CO2, and reading the associated pressure above. There's no "2.16" but it's between 2.10 and 2.20 volumes.
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