Difference between forward momentum  

"Momentum" is simply movement (thrust) in a direction.  ETL is an aerodynamic principle related to helicopter flight that explains how a rotor disc becomes more effcient and, thus, provides greater lift and thrust at a given power setting.  I.E., at, and after, ETL you need less power because the rotor disc is much more efficent as it stops recirculating air and leaves those vorticies behind the disc.  "On topic" enough for you? 

BTW, it's still a stupid question and the fact that you can't seem to grasp the really basic and simple answer really bears witness to the actual issue; and it isn't "momentum vs ETL." 

A product of the more recent public education system?  Little Johnny, does is 2 + 2 equal?  6!  How, so close!  Don't you feel good about yourself?