I suspect that the aircraft would have been instrumented with `strain gauges` `g meters.,etc and a series of gradually increasing manoeuvres carried out to determine actual stresses involved ,in support of the designers calculations
Sycamore,
I agree, the chap concerned mentioned that the trial comprised a number of dives amongst other manoeuvres. Unfortunately, I was a young FZ eaves dropping on an adult conversation so not in a position to question him further.