My understanding is they filled the super lightweight tubular frame with compressed nitrogen. There was a pressure sensor built into it so that if the frame cracked, it would illuminate a light on the dash.
They used a similar system on metal rotor blades to detect spar problems on helicopters. This was before composite blades.
They also used it for piping to the oil cooler.
This was truly a remarkable car that was a real handful to drive but man it could fly.
Jo Siffert had practically a nervous breakdown from driving one. A very exhausting car to drive but truly ahead of its time in many areas.