The Salvo trigger is a trigger that allows you to load multiple rockets then fire them off at the same time. Holding down the left mouse button will load the rockets, which you can see a circles below the crosshairs. When all your rockets are loaded in (you will know this because no more dots are appearing) you release the trigger to shoot all the rockets out in a horizontal cone shape. You will have a number of seconds until the rockets release by themselves. Also, using the Corkscrew Propulsion causes all rockets to be combined in the same corkscrew.
When using salvo with a Cage, Heavy, or Toob barrel, the trigger works similarly to Charged. As you hold down the trigger, you increase the distance and speed that the rocket will go. This DOES affect the MAX damage of the rocket.