A soap sample is a bottle of oil pulled from the engine after you have given it a run. Its then sent off to the lab where they do things to it. And send you a report on your engine. The report is scary accruate. They can tell you if you rings are leaking what components are shedding metal, if a bit of your engine is overheating locally etc etc.
And neither did the finish airforce and they have just had to change 4 engines. It will be interesting to see what they find when they boroscope the test aircrafts engines. I suspect for pistons you won't see anything in most cases until the next time you pull the cylinder head or something wears out way before its meant to.