Each back brace is a "sandwich" of sapele (the dark wood most of the chair is made out of) and a core of ash. Ash is a very supple and bendable wood (think English longbows). The back braces are floating in the frame of the chair, so when you lean back in the chair they will bend to fit your back.
To get the bend exactly right, I first cut the curve out of a thick piece of ash (with holes so I can insert c-clamps), then I made a rigid clamping stock out of several thin pieces of ash that I clamped to the curved wood form. Then it was as simple as cutting a bunch of 1/8" strips of sapele and ash, and gluing them up like a sandwich, sticking them in the form, and clamping them super tight:
View attachment 625501
I needed 7 back braces for the chair, but I made 8 just in case I needed an extra. Two at a time, so it took four days to clamp up the back braces (letting them set for 24 hours before I took them out of the form). I worked on the frame of the chair at the same time.
View attachment 625502