This is related to the phenomena that tea leaves tend to group in center of cup when you stir it with spoon. Due to centrifugal forces the level of water near

This is related to the phenomena that tea leaves tend to group in center of cup when you stir it with spoon. Due to centrifugal forces the level of water near the border is higher that at the center so the water start to flow from borders to center at the bottom of the cup. This speed up heat exchange as the water gets constantly mixed in addition of usual thermal exchange flow.