The decoys are a copy of the land units surrounding the general at the time the ability completes, in a ghostly-looking form. Their sole purpose is to distract the enemy, and maybe scout dangerous areas without losing units. However, they have a very low viewing range.
To the enemy, they appear as normal units would except that they do not inflict any damage. A human player might notice this and order his troops to attack specific units rather than others, but the AI has no way of telling. The decoys will last for as long as their "health" allows, or until the blue bar above them runs out. They will die in a single shot when attacked, including all sub-units of an infantry unit. Additionally, if an enemy detector is nearby, they continually lose hit points, similar to attrition. Decoys are also affected by attrition, just like regular units.
If a decoy unit is double-clicked on, all decoy units within range will be selected instead of just all decoy units of that type. No other units will be selected.
Decoys can garrison inside buildings, where they can heal their hit points. However, they do not contribute to the building's attack, nor do they take up garrison space.
Using this ability can offer sufficient advantages if the enemy is unaware of the decoys. This can allow your real units to inflict damage or infiltrate important areas while the fake units can distract and suffer no damage in the long run. Decoys can also divert some enemy fire away from your real units if placed in front of your units when attacking. Although they die to a single attack, this can still buy some time for your units to survive and attack, especially against longer-cooldown units such as Machine Guns.