The old structure was made out of aluminum tubing and it instantly collapsed when aero load was developed. The new structure is made out of strong and stiff chromoly tubing and attaches to the engine cradle/cross member.
This makes the BNR32 a flexy flyer. It was also causing vexing problems like severe flex when the load of the steering box was applied to it. Since time attack rules precludes loping off the front of the car and making it a tube frame, reinforcing the unibody would have to do.
Brian built these nifty chromoly braces to triangulate the frame rails to the solidly mounted transmission and subframe/engine cradle. Now the chassis will be much stiffer in this area, the engine and trans can be effectively structurally coupled to the chassis in more places, and the aero load is spread out over a wider area.
With the steering box now partially mounted to the cage and the frame rail being reinforced, things are a lot better.