I thought it might be a curved brace similar to what Steve Klein uses in his guitars, a composite carbon-fiber graphite and spruce brace which forms a parabolic (or maybe semi-elliptical) arch springing from the back linings and touching the X-brace at the center of the soundboard. This is alleged to allow for lighter soundboard strutting and hence to improve the tone.

If it is actually a curved brace glued down onto the soundboard, I would be interested to know how he achieves the curve. My assumption would be that he laminates it up from multiple thinner lengths of spruce.

Thanks for enquiring amnd I look forward to hearing the answer.

Murray