Reheat
Reheat or afterburning as it is sometimes known, is a method of giving a large increase in an jet engine's thrust over a relatively short period. It involves injecting large quantities of fuel directly into the engine exhaust and igniting it, making use of the unburned oxygen left over from the main combustion process.

Reheat gives large thrust gains relative to the small increase in engine weight. Some jet engines can almost double their thrust with the use of reheat but the downside is extremely high fuel consumption. As a result, reheat is used mainly to provide extra thrust during take-off and during combat maneuvers for military aircraft. It is not practical for reheat to be used for prolonged periods. Because reheat is gives increased performance by sacrificing economy, it is used exclusively on military aircraft with the notable exception of Concorde.
Reproduced with kind permission of Rolls-Royce plc
The components of a reheat system are :- a high pressure fuel pump, a fuel metering unit (often combined with the fuel pump in one unit), a fuel injection manifold, an ignition source and a variable exhaust nozzle.

The fuel is introduced into the jet pipe downstream of the final turbine stage and ignited. Although early reheat systems gave a fixed amount of additional thrust, modern systems are variable with the pilot being able to select the degree of reheat he requires. Reheat is normally only available when the engine is already at full power. The pilot pushes the power lever past the max power stop (or detent) into the reheat range.

Because the amount of reheat is variable and there is a need to maintain the correct pressure ratio in the jet pipe, the exhaust (or propelling) nozzle is also variable in size, depending on the degree of reheat selected.
Quicktime video clip of Eurofighter Typhoon on take off run
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