There's three different arm positions in a single picture. That doesn't happen in the blink of an eye.

It's a lot faster than you might be expecting. I found it helps to visualize it in person. Go to a mirror and start with your hands together like in the right side mirror. Now let your arms down naturally, to the position in the left side mirror. If you don't move your arms at the same exact time, one elbow will still be parallel to the floor while the other elbow has extended already, just like in the middle position.

Thus, we can tell that the camera compiled the image from right to left.