This is the image, taken directly from your mirror adjustment video. The red triangles (added by me) show the “blind spots” you have when using just your mirrors, as adjusted in this video. If you fail to check these blind spots before making a driving maneuver, you could easily kill someone. You have to turn your head to check these spaces. The act of turning your head is the literal definition of checking a blind spot.

Feel free to back out of this discussion, but be sure to check your surroundings before going in reverse.