it's so interesting how the partial drawings are like silhouettes. I wonder if there's some lighter lines that aren't being picked up in the video

I wonder if there's some lighter lines that aren't being picked up in the video

No. This version of these cuts are set to the timesheet, and where there's inbetweens, they use a blank drawing instead of having the key-frame be there longer.
Its a bit hard to explain with writing, but if A1 and A2 are 6 frames apart, and there is a 3 frame inbetween between A1 and A2, the inbetween's 3 frames will be white on this video.

[A1 - - - inbetween - - - A2 - - -]

yeah I know what you mean.
What I'm talking about is that the video quality is low and overexposed, so we may be missing lighter pencil lines

MarcHendry said:
it's so interesting how the partial drawings are like silhouettes. I wonder if there's some lighter lines that aren't being picked up in the video

Those partial looking drawings are called 中割り参考('nakawari sankou': sometimes just 中割り/'inbetween' for short). For movements that are hard for your average dougaman/inbetweener to inbetween, the animator sometimes partially draws the important imbetween frames to serve as a guide for the inbetweener. This makes their job easier and helps the key animator get a result closer to what they intended.
If you look closely, you can see 他中割り(hoka nakawari) written on some of those 'partial drawings'/nakawari sankou, which essentially tells the inbetweener to 'inbetween the other parts' of the drawings since as the name suggests only some of the inbetween is actually being drawn.
Lastly just wanted to mention while key frames are also marked with a circle(like ①), partial drawings/nakawari sankous are also usually marked with triangles(△) and hiragana/katanaka letters are written inside(あいうえお・アイウエオ）instead of numbers, so its easier to differentiate them from key frames. Some people use decimal numbers instead of hiragana/katanaka symbols though

The reason the lines are faint/poor quality on the video is because these genga/key animation videos are made using something called a QuickTime Scanner(QTS for short) which essentially allows analog animators to quickly check their work in a video formatーmuch like how most digital drawing software have a timeline along with a playback feature so digital animators can watch test their work. Sadly its not really good at picking up specific lines though and that leads to stuff like this. This genga special video itself seems to have been from the original DVD release also and those were never known for being especially high bitrate encodes.

yeah we do a similar thing in western animation, but it's just called a "partial drawing".
It's usually less about the movement being hard for the inbetweener, and more that the animator has something specific they want to control, while the rest of the body is a more self-explanatory inbetween.
Although the jobs are a little different for us at cartoon saloon too, we don't have people whose entire job is inbetweening. Rough animators do almost every drawing, and anything that's missing is handled by cleanup.