All damage dealt with rockets is doubled. Each attack has a 20% chance (regardless of the Proc Coefficient) to fire a rocket for 150% damage as Physical (increased to 300% by the skill's own bonus).
Ballistics is a Demon Hunter passive skill, unlocked at level 55.
This skill affects all types of rockets, including its own bonus launches, regardless of the Elemental Damage type. The skill's own rockets are fired at a random enemy within line of sight, not necessarily the current target of the attack. An attack or cast does not have to be damaging for the bonus rocket to be fired.