The other amazing thing is that all the recordings of him are done with two random people they just dragged in and did on the fly.

They seem so perfectly weighted and with exactly the right increase in difficulty that I assumed they must have been heavily edited / selected.

But basically he kept his methodology secret - literally locked in a safe - because he didn’t trust anyone after his experience in ww2.

It wasn’t until he was really old that someone convinced him to make the recordings. And the tapes are just that.