Flushing upstairs clears a stopped toilet downstairs
The hydrostatic head water pressure on the sewer plug from the downstairs toilet is roughly 1 psi. The hydrostatic head from the upstairs toilet/drains puts roughly 6 psi on the plug. That is about 7 pounds force at the plug from downstairs or 42 pounds of force from upstairs ( with a 3" I D sewer pipe).