A guess: The jam sucks water out of the air, lowering its volume, producing vacuum, which is sufficient power to run the lid down the threads.
There would likely be a correlation between level of jam in the jar and speed of closing / ability to do it at all; a larger volume of air will take longer to shrink and have more elasticity so it might only work for almost full jars.
There would likely be a correlation between level of jam in the jar and speed of closing / ability to do it at all; a larger volume of air will take longer to shrink and have more elasticity so it might only work for almost full jars.