Thanks to everyone above to have given me my refresher course...
Now my turn to open the "squawk box"...
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Thrust reversers achieve TWO (2) things...
When deployed (i.e. "idle") - they cancel (or "block") the residual idle thrust...
With power - they vector the thrust in a somewhat "forward" direction resulting in a braking vector...
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There are two types of reversers...
(1) Clamshell type - look the picture above of the KLM Fokker twinjet...
This type is used for low bypass engines, and mounted on the turbine exhaust.
(2) Cascade type - look at the picture of the 737 (?) above...
This mostly used for high bypass engines, to deflect FAN air towards forward direction.
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In the 747, originally, each engine had a set of FAN and TURBINE reversers. The turbine reversers (on the exhaust) were often subject to malfunctions. Most, if not all airlines decided to suppress the TURBINE reversers, and kept only the FAN reversers - Think of it this way = very little thrust comes from the turbine exhaust, most of the thrust comes from the fan anyway. Very little "effectiveness" got lost with this modification.
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For the old timer crews or passengers, I flew the DC8-60/70s, in which the inboard reversers (engines nš 2 and 3) could be use IN-FLIGHT as air brakes (on the DC8, spoilers/speed brakes cannot be used in flight). The use of reversers in the air was quite "drastic" with noise and vibrations, so that we made a cabin announcement anytime we had to resort to the use of reversers in flight.
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Happy contrails