2D subdivision surfaces - How to arrive at $B^1 = \large{\frac{A^0 +6B^0 +C^0}{8}}$?

Question

In the video "Math and Movies (Animation at Pixar) - Numberphile", at around timecode 11:45, the following is said:

Using the $1$, $2$, $1$ rule, you can see that $B^1 = \large{\frac{A^0 +6B^0 +C^0}{8}}$

How does one arrive at this please?

I wonder where the $6$ and the $8$ come into the picture.