ANIMATION: elongated inbetweens

On page 96 of his book, The Animator's Survival Kit, Richard Willams introduces elongated inbetweens. The pounding on a door is one example he uses.  In addition to including an elongated inbetween (Drawing 2, below), he also includes a couple of different timings for the animation.

Here are the three drawings used:

Here is the animation included the two extremes (Drawings 1 and 3) shot on twos:

Drawings 13 13 13

The elongated inbetween is included in the next animation and it and the extremes are shot on ones:

Drawings 123 123 123

The last animation, the inbetweens are shot on ones; the extremes are shot on twos:

Drawings 1 23 21 23 21 23 2