Something to remember is that the thrust is NOT directed exactly forward (i.e. 180 degrees from the reverse axis). Once the aircraft touches down, electrical switches on the landing gear enable the thrust reverse control system to engage. The pilot brings the engines to forward idle, and pulls up on a set of smaller levers forward of the actual throttles. This causes the "buckets" or "sleeve" (see below), to swing back into the engine's thrust stream. Once the pilot sees that all engine thrust reversers are engaged, through a set of indicator lights, he advances the normal engine throttles to bring the engine power up and the reverse thrust force higher.
On the T-Tail aircraft, thrust reversers are large "buckets" of metal that mechanically swing back and "clamshell" into the exhaust stream of the engine, deflecting the thrust maybe 10-20 degrees max forward. These "buckets" are actuated either pneumatically (using air bled from the engines) or hydraulically (using a self-contained hydraulic system).
On underwing engines, the principal is the same but you really can't see the reversers working. Usually a sliding sleeve on the engine slides aft, which closes off the buckets inside the tailcone, deflecting the exhaust to the sides and forward, but nowhere near directly forward, more like 10-20 degrees....
Some of the myths of thrust reversers are:
(1) the engine stops, and spins in reverse, directing thrust forward (NO! not even close)
(2) the compressor blades in the engine are pivoted to direct the thrust forward (NO! although some engines do have variable stator vanes, these are used to schedule the airflow through the engine)
Hope this helps!