Option. Intervention is in case of a power emergency.

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Germany has most of its winter emergency power reserves: regulator
Germany has secured 2,022 megawatts (MW) of the 2,540 MW emergency electricity capacity it needs to get through the winter, the energy regulator said on Monday, confirming a forecast that possible shortages were being managed.
http://www.reuters.com/article/2013/09/16/us-germany-power-r...

W odróżnieniu od "spinning reserve" czyli rezerwy gorącej.
Np. tutaj:
http://en.wikipedia.org/wiki/Operating_reserve

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Note added at   23 godz. (2014-06-22 11:49:29 GMT)
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Zarówno gorąca, jak i zimna rezerwa mają charakter interwencyjny, przy czym w przypadku zimnej "interwencja" jest głębsza i poważniejsza, gdyż wymaga uruchomienia dodatkowych generatorów.
"Spinning and supplemental reserves (collectively referred to as contingency reserves) are used to maintain the balance of supply and demand when an unexpected system event occurs. These reserves provide capacity the system controller can call on with short notice to correct any imbalance. These reserves can come from the supply side (generators) or from the demand side (load curtailment by turning off large electrical consumers to reduce demand immediately – note load can only provide supplemental reserve). Spinning reserves are the fastest acting contingency reserve. Generators providing spinning reserves are synchronized to the grid (the turbine is “spinning” but not generating power), this unique feature allows the reserve to be provided very quickly. In addition to the ability to respond very quickly, spinning reserves also provide frequency support to the AIES. Supplemental reserves on the other hand are not required to be synchronized to the grid and are slower to respond when called upon."
http://www.aeso.ca/gridoperations/19607.html