First you have to realize where this lie started. Russel wrote in his book Time at hand that the end and Armageddon would take place in 1914. When that didn't happen the new leader Rutherford said that the end did come Jesus came invisibley and the gentile times were over. As time has passed they change the meaning of things and change things around to hide the fact that everything they said was wrong.
They could not admit that when he Rutherford said in 1917 that this generation would not pass away was wrong so they had to again change it by saying new light and the generation is an overlapping thing. There is no place in the bible that supports this idea. The bible makes it clear in several places that a generation is roughly 40 years. Mathew chapter 1 lists generations etc.
The whole doctrine by design is to continue to mislead the ignorant jws.