When you do unbendable arm you cant let your arm relax completely it just bends. Given the great threads about the fascia system perhaps we could re translate 'extend ki' as 'Relax muscles - engage total body fascia system'.
FWIW Rod Kobayashi "retranslated" a couple of the principles for us. So instead of "extend ki" he said "let your ki flow". The idea to get that feeling throughout your body of "connectedness" to allow the "ki" to move. Instead of "weight underside" he went with "settle down". From what I understand he liked the emotional connotation of "settle down" as well as the idea of everything simply settling in to its "proper" place. So it's not about "getting low" as much as a way of standing allowing things to settle in to their proper places. So add those in to the idea of "controlled" relaxation.
I've always found it interesting that he translated these things differently from the "official" ki society translations. He was bilingual and apparently felt that the translations didn't always quite convey the meaning correctly.
Just a gigantic fwiw. Interesting glimpses looking back... Seeing through new eyes and all that. I'd love to have a time machine and go back 35 years and ask a few questions...