Killing a target increases your health by 6% (for build revision 1,7 and higher). Older builds may only give 4% increased health. When a weapon has the Sustained Talent, this may proc on skills such as the Seeker Mine achieving a kill. This means that on certain Skill kills, the talent could proc without the weapon itself performing the kill. The reliability of this mechanism is unknown.
The required firearms and stamina stats to activate apply at nominal Gear Score 256. Lower nominal GS requires lower stats.