Tracer Missile grants Tracer Lock, increasing the damage dealt by the next Rail Shot by 6% or reducing the activation time and increasing the healing dealt by the next Rapid Scan by 20%. Stacks up to 5 times.
Patch 3.3.0 (22 Jul 2015): Tracer Lock now reduces the activation time and increases the healing dealt by the next Rapid Scan by 20% (instead of Healing Scan).
Patch 1.4.0 (26 Sep 2012): Each stack of Tracer Lock now additionally reduces the activation time of your next Healing Scan by 20% per stack.
Patch 1.3.0 (26 Jun 2012): Tracer Lock and Light 'Em Up have swapped positions in the Skill Tree. Tracer Lock now requires Tracer Missile and Light 'Em Up now requires Tracer Lock.