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The sail switch is a safety switch to prove that the combustion power blower is running.This switch must prove before power is sent to the gas valve.On commercial boilers they call it a air proving switch but it does the same thing however works different.The sail switch is probably just a metal tail on a microswitch that is located in the air stream of the combustion air fan.If the fan runs the little tail will catch the air and move,thus closing the contacts on the microswitch.If the switch is defective or the sail part breaks off;or worse muddoppers or wasps build a nest in the airtube the switch will not close and thus no power to the gas valve.If you feel air on the exhaust blower side of the furnace and no heat you probably first would check the airswitch.Hope this helps shed some light on the subject.
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