What is the theory (in layman's terms) behind the thrust augmenting tailpipe? Is is simply a divergent duct increasing the exhaust pressure? if so, there is always a penalty to be paid and surely this would be in terms of exhaust gas velocity (and hence top speed).

I am still a believer that you don't get something for nothing.

Also, the thrust that is being measured by most people is static thrust. Does anyone know the magnitude of the benefit these "augmenting" pipes actually have in the air?