Decryption is a ruse in R.U.S.E.. It is a revealing ruse
Effect
The decryption ability reveals all enemy movements within a sector. Enemy units affected by the ruse will have red-and-black arrows hovering over them, and their orders are represented by red arrows stretching from the unit to its intended destination, even if that would lead the unit well outside of the affected sector.
Unless a Recon unit has Line-of-Sight (LOS) with any of the enemy units, the movement will be represented by either Heavy or Light unit 'tokens' bearing the insignia of the unit's corresponding faction, since Decryption does not reveal the identity of the unit (i.e., whether a unit is a Panzer III or a Puma). This also means that Decryption can be misinterpreted, especially if the enemy uses Reverted Intel as a Counter-Ruse.
Strategy and Tactics
Decryption's most obvious use is to monitor the movement of your enemy, but it can serve less immediately-evident purposes too. For example, the ability to see your enemy's orders can be used as a gauge of effectiveness for whichever ruse you currently have in play. Say you have a Decoy Offensive in play; If the majority of your enemy's orders are headed in the direction of the decoy troops, you can assume your opponent believes the ruse to be an actual offensive rush.
History
Decryption was in fact a large part of the war effort in World War II. In R.U.S.E., during the campaign mission Kassarine Pass, Colonel Campbell makes reference to 'Bletchley Park'. Not only did Bletchley Park (a.k.a. Station 'X') exist, it was the nerve-center of the Allied Force's cryptanalysis efforts. Decrypting German communiques with stolen and captured Enigma and Lorenz Cipher-machines, a veteran of Station X claimed that the high-level intelligence (codename: 'Ultra') "...shorted the war by no less than 2 years, and probably by four." Moreover, without Ultra, it is uncertain how World War II might have ended.