Why does this guy's automatic air cannon have 2 valves?

[Chauvo]

From what I understand, the bb with a spring near the trigger is what makes it automatic... So what is the purpose of the sprinkler valve?

[jackssmirkingrevenge]

The sprinkler valve is the main valve but it needs to be piloted to fire. By having a pop-off valve as the pilot, it will be piloted at certain intervals effectively giving full-auto fire.

I did this with a eBay Quick Exhaust Valve coupled with a pop-off safety valve:

[youtube][/youtube]