Sebolino, one of the best metaphors for a jet engine (or any reaction engine including a rocket) is a balloon full of air (like a children's balloon). When the balloon is sealed, the air inside exerts an equal pressure on all the inside surfaces. Therefore, the balloon tends to stay put if you ignore gravity and external airflow (wind).
If you open the nozzle (opening) of the balloon, pressure is released at that location. At the opposite side, the pressure is still exerting a force. The result is an inequality of forces acting on the inside of the balloon. The resultant force propels the balloon away from the opening. You have probably tried this as a kid.
Reaction engines work the same way. The pressure of burning, expanding gases is released at the back (exhaust). At the front of the combustion chamber, the pressure is contained by airflow entering the engine. The inequality of forces means the engine wants to move forward.
When reversing, the pressure is released in a forward direction instead of backward (by redirecting the exhaust). The result is that the engine wants to move backwards since the net pressure on the engine is greater in that direction.
However, engines are designed to move forward, so they are more efficient in normal thrust regimes than while reversing, meaning you get more power in the correct direction.
Simplistic explanation, and maybe someone can help me out. Anyway it's the best I can come up with.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo