I will say it is to make sure the frames still meet or exceed expectation special after improvement or upgrade. They pickup a frame each year for the testing. We had photos for another EK frame done the test last year (don't ask me which MSN it was because I don't remember but I am pretty sure FabienA380 or XWB has the info). The trailing cone is a sense to measure certain kind of criteria.
Please correct my answers if you have more accurate ones
I looked up what a trailing cone does. It is used as a refernce (static) preassure sensor as reference to the ones on the aircraft. The cone is just used to stabalize the hose, which is the measuring part.