How does the reverse thrust works on aircraft?

Reverse thrust are used to slow down the aircraft after touch down on the runway.

We slow down the aircraft by means of three components 

Primary- carbon breaks

Second- reverse thrust

Third- by the aerodynamic flight controlles like spoilers.

We have two words in reverse thrust first one is reverse the turbine directions are reversed to opposite direction And as you might of known increases the thrust for breaking action.

So let's look at the closer at the touch down the small opening of flaps is open during touch down as you can see in above picture.

As you can see clearly the as this flap like configuration opens then the outside air enters into the turbine and changes the direction of the original air.

Now this reverse thrust don't opens automatically we have to pull this-

We have to lift of this little livers which actives the hydraulic pressure and then the reverse thrust opens from the actuators.

But the pulling out of this levers will give the only ideal reverse we have to pull both thrust and levers to the off position like-

Using the reverse thrust it easily decreases the landing distance, vary between the aircraft weight and environmental factors

As you can see in the above picture the b747 consumes the less Runway length by having it's four reverse thrust and environmental criteria and it's weight.

Using of idle reverse thrust are mandatory after touch down for all airlines. For stabilize the aircraft on the runway center line and reduce the use of the break.

Landing of the aircraft on snow covered runway and wet runway there are chances of aircrafts skidding over the runway that's why use of idle reverse thrust is mandatory instead of using Break system.

Use of full reverse thrust is not permitted at some airports because of noise abenments concepts

As you can see in the above notice it is clearly mentioned that avoide using full reverse thrust at JFK airport.

Use of Reverse thrust to it's idle position is at speed of 70 knots for an a320 aircraft is mandatory as per manufacturer.