In Wolf Lowenthal's book There Are No Secrets, the author presented a situation in which Cheng Man Ching had asked the students to push hands without using the arms.
The students were using tension.
They were forcing a result.
Cheng Man Ching wanted the students to push hands in a different way.
However, he was not prepared to explicitly tell the students what to do.
Years of fruitless practice bore no result.
After Cheng Man Ching's death, the author was watching a video of the instructor pushing hands when he suddenly saw how to push hands correctly.
In hindsight it seemed obvious.
Cheng Man Ching was not keeping secrets. In fact, he was showing the very skill all the time. But his students were just not ready to see it.