Actually, there is very little to explain. If a flight is conducted in a flying club and the pilot is an instructor then potentially any flight could legally be a lesson and it is up to the authorities to prove it isn't. That is very difficult, which explains why certain "non instructors" manage to get away with illegal lessons. Unless you are carrying out blatantly illegal activity, you have nothing to worry about except perhaps instructor liability, which few carry. You have 10 times more chance of being sued by the family of the deceased than for wrong doing by the authorities.