When an iPhone is plugged in and syncing, the status bar shows a message like:
Syncing "Phone" (Step 2 of 6)
On the left side of the bar there is this icon:
It darkens when clicked, which implies that it's a button. If I had to guess from the icon alone, I would say "rotate clockwise", but that doesn't make sense in this context.
Hovering does not display a tooltip. Clicking it does not have any visible effect.
What is this button?