the description given for ishi's practices are just very general. to his mind (as it was described), the bow should bend in an even "arch at all points". when the bow was thinned out and shaped ishi would apply sinew, at which point the bow would be left for some period of time to dry. during this time the bowstring was made. when the bowstring was done, and the sinew backing had been drying for some reasonable amount of time, the bow would be braced.
pasting directly from the earlier reference (Yahi archery, 1918 13, 3, 103-specifically on 109:
Usually he made his string with a tapering
extremity which rendered it easier to fasten. Then he cautiously drew
his bow and observed its bend. On cold days, Ishi warmed his bow
over a fire before attempting to brace it. The ideal bow, to his mind,
curved in a perfect arch at all points, and at full draw represented a
crescent. The center bent with the limbs and was the bow s weakest
point. A forty-five inch bow he drew twenty-five inches. No yew
wood could stand such an arc without backing. In fact he broke
two bow-staves, testing them at my request, prior to the application of
sinew.
Where the contour showed the bow too strong, he filed or scraped
it on the belly side, thus gradually distributing the bend evenly along
the bow.
SOM