The gas is usually always "in" the valve.
The gas is stopped or allowed to pass through the valve to the burners by internal 24V solinoids powered by the board and monitered via a main burner flame sensor system. The gas also passes through a regulater in the valve to reduce the gas pressure from the supply side to the output.
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If you are talking about a direct ignition system then a spark is generated by the ignition box and a micro current is sent back to the box sometimes via a body ground.This keeps the 24 volts on the valve until the stat sees the set temp reached.
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