The controversial (apparently) difference in scores between a fall after a full turn and a fall due to an under-rotation.
In the case of a full turn and fall, the Deductions will be deducted one point and the GOE will be reduced, but the basic points of the jump attempted will be added.
In case of a fall due to under-rotation, the base points will be downgraded.

Some people say that it is important not to fall on a jump, and that a fall is a failure jump, whether it is or not.
In case of a fall after a turn, Deductions will be deducted one point and GOE will be deducted, but the fact that the basic point of the attempted jump will be added means that the points will be deducted for the fall, the GOE will be deducted because it does not look good, but the quad is still a valid jump, so it will be recognized and full basic points will be given.
For example, if a skater lands on both feet without rotation, no points will be deducted for deductions because he/she did not fall, and points will be deducted for GOE because it does not look good, then, can we recognize it as a quadruple? If you have to ask yourself, "If it is not a quad, then it is not a quad," don't you think? So, the basic points are deducted.
I think that both of them are very reasonable in theory.

Anyway, if you don't have to fall down at any cost, wouldn't it mean that you have to avoid doing under-rotations or landing on both feet from the beginning, or avoid doing difficult moves or difficult moves from the beginning?
Your technical progress will be stagnated.

Come to think of it, there was a former skater who didn't jump loops....

I made a page to summarize Yuzuru Hanyu's controversial jumps in FS of China Cup. It's painful to watch.
⇒Yuzuru Hanyu - Jumps - ISU GP Lexus Cup of China 2014

Looking at the landing of the jumps that fell, I thought that many of the jumps fell because they could not hold their stance after turning and landing.
It doesn't look as if you lost your balance by being under-rotated.

The first half is until the single flip, and the second half is after that, which is 1.1x. Both combinations are successfully jumped in the second half.

As for the PCS, I'll refrain from mentioning it because I'm still a gnat, but I don't think it is being missed as far as under-rotation is concerned.
It is just my personal opinion.

Should we take a under-rotation for Hanyu's tumbling jump?

Hanyu's 4T (went around and fell)

Takahashi's 4T (fell due to under-rotation)

All Japan Figure Skating Championships 2011 FS


ISU Grand Prix Final 2012 FS


ISU Four Continents Championships 2012

※However, Takahashi's fallen 4T, the top two are not under-rotated.
Scored by
4T 10.30 -3.00 ←Hanyu's Cup of China
4T 10.30 -3.00 ←Takahashi's All Japan Championships (Winner of the legendary Free 3 fall)
4T 10.30 -2.70 ←Takahashi's Grand Prix Final
4T< 7.20 -3.00 ←Takahashi's Four Continents Championships

Who should get the under-rotation?

obvious.

Also, some people say that you just jumped but didn't land on the ice, but isn't this what you mean by a jump that you just jumped but didn't land on the ice?

Hae-Jin Kim's 2A

There was no landing motion and it was more of a fall than a tumble.
I used it because it is a very clear example.