The sail switch needs to be in the open position. At the end of a heating cycle the sail switch needs to return to the open position to reset the control board so the furnace can start another cycle.
If the sail switch sticks in the on position after a heating cycle the furnace control board will sense that the sail switch is in the wrong position (ON) and will not let the furnace start. This is a necessary safety procedure to insure that the furnace goes through its proper purge cycle before firing . On many heating devices there are redundant safeties such as dual temperature hi limits ,gas valves ,proof of air flow ,stack dampers ,to insure that no one safety switch is defective , and lead to a possible explosion or monoxide issues.