It works by splitting the mouth into two sections, which can be distorted by changing the viewing angle. In this example the creasing goes through the centre of HM's eyes and nose, so these don't distort very much. However the mouth does shift from upturned to downturned as the viewing angle changes.
Filming insight here
And if anyone is interested, these bank notes feature a holographic emblem, watermarks, interweaved metallic-strip, and high-quality printing - they're rather hard to forge.