How do thrust reversers in high-bypass turbofan engines counteract initial thrust? I've been searching the internet for awhile and haven't really come across a good answer for this question. So:

I have a basic understanding of the principles of flight (thrust, lift, drag, etc). But one thing that has been confusing me is how exactly a thrust reverser on a high-bypass turbofan engine works. So from what I understand, the large fans at the front of the engine will provide the majority of the thrust. Air coming through the inlet nozzle produces an action-reaction pair on the fan blades to produce this forward thrust. So upon deploying the thrust reversers, the air that has already produced a forward thrust on the fan blades now moves back through the bypass duct to strike the thrust reverser, thus redirecting the air forward by creating another action-reaction pair to slow the aircraft. 

So it just kind of seems to me that the thrust reverser just cancels out the thrust from the fan blades. Or is the air coming off the fan blades is accelerated so that by the time it hits the thrust reverser the forward directed thrust is greater than the thrust generated from the fan blades, thus producing a net reverse thrust?

Or maybe I'm just looking at this wrong and need to think of the entire engine as a closed system so that the air pushed backward from the fan doesn't produce an action-reaction pair (and thus thrust) until it exits the engine?

I'm just kind of confused on the specifics of the physics of the thrust reverser (where the action-reaction acts).
There are several ways to describe thrust, which amount to the same thing. Very broadly, it's described by Newton's third law: if A moves forward, B moves backward with equal momentum. On a closer level, it comes from pressure differences across the surface of a body. Even closer, you'd be looking at viscosity and velocity, laminar and turbulent flow, boundary layers, analytical equations.

But all these ways come to the same result: if one, applied correctly, predicts result X, the rest can't predict an opposite result. In the most basic view, the reverser's thrust comes from pushing air forward. As long as the ultimate result is air being accelerated in some direction, however it happens, the thrust will be in the opposite direction. 

If you want to get into the mechanics (which isn't quite necessary), the exhaust produces high pressure between the engine and the thrust reverser. This pressure acts on the thrust reverser's inner surface. The resulting backwards push is indeed greater than the fan's forward thrust.