I suspect it's just gravity pulling the lid down along the groove. Kind of like going down a spiral slide. With metal on glass it probably has relatively low friction. I've seen nuts thread themselves down bolts as long as a foot, with just the slightest initial push, in a similar manner.
It's amazing to see but I think it's just like a ball rolling down an inclined plane. A screw is just a twisted inclined plane. For various combinations of friction and angle gravity can make it happen.
This is exactly it. I'm not sure why people are surprised by this, I've seen it happen plenty of times before. The lid never tightens itself, it literally is just falling into place along the grooves.
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.
What kind of weird watery jam do you have? The air suck must be enormous, to spin a lid down quickly. Are you sure your not spreading desiccant on your toast?
For those that don't know: Concentrated sugar solutions are indeed hydroscopic.
So it will indeed absorb water out of the atmosphere.
It's still doubtful that it will do it fast enough to create any significant pressure differential though :)
Could also be cold jam. When the lid is open, the air inside the jar would be room temperature. As soon as you put the lid on it, the air starts cooling down because the jam is cold, which would lower the pressure, and then the outside air would push the lid down, turning it.
Even if this was the case I doubt it would create a strong enough vacuum to move the lid. I think the threads just slope downwards a little more than other jars and they let gravity do the rest. Maybe the lid is lined with a slippery rubberized material that creates a seal, which would minimize friction allowing the lid to spin with minimal force.
Edit: if someone has one of these jars, tilt it a little and then try lid trick. I bet it won't work.
