Salvaging is about tech the player cannot get currently, not tech that just hasn't been researched yet, and is based on techs the player has a direct link to.
For example, it the player has Polysilicate Alloy and has failed the research for MagnoCeramic Lattices. if the player is fighting a enemy that is using Quark Resonators, the player will not be able to salvage the MagnoCeramic Lattices tech from those ships and won't be able to salvage Quark Resonators until after MagnoCeramic Lattices is salvaged. 
And as a final example, the enemy has Phasers on their ships. The player hasn't researched that tech and has not even done the feasibility study. However the link would be feasible for the player. This means the player will not get Phasers as a salvage project, since it is possible for the player to research Phasers.
To have a chance to salvage techs, ships with the Repair and Salvage section only have to be in the same system when combat takes place. They do not have to be deployed into combat.  Repair and Salvage ships must also be in an active fleet, not in reserve.
Salvaging can come from 2 places: Ships or colonies. When the player destroys a ship they have a chance to salvage only the weapons or armor that the particular ship was using. However when the player destroys an enemy colony, they have a chance to salvage any non-race specific tech they have a link to.
Base Salvage Chance
Each Repair and Salvage ship (or every ship for Zuul) has the following base salvaging chance:
- Zuul (with a Repair and Salvage ship): 6%
- Zuul (without a Repair and Salvage ship): 3%
- All others (with a Repair and Salvage ship): 3%
- All others (without a Repair and Salvage ship): 0%
The roll for each ship seems to be independent of each other, so bringing 20 R & S ships will result in 20 rolls at 3% each instead of one roll at 60%.
Other Salvage Modifiers
Capturing a ship will improve the salvage chance by a small amount. 
Different government have bonuses or negatives to their salvage percentages. For example, Communalism and Mercantilism both get a boost, while Junta gets a penalty and Plutocracy gets a larger penalty.