Freerunning is tactic used most often in Quarantine.
Freerunning involves actively moving around a map, instead of holding one position for too long. It differs from rushing by being tailored more for outrunning and eliminating zombies, rather than human enemies. It requires the player to both move and react quickly in order to outmaneuver the infected, as well as to understand the map.
The upsides of freerunning include: being able to use the tactic efficiently without teammates, and being harder to locate (due to actively moving around the map, and the infected focusing on popular saferoom and camping spots).
The downsides include: lack of friendly support, lack of saferoom security, and an overall increased chance of failure.
Freerunning Outside of Quarantine
While most people identify freerunning with Quarantine Regen, it also has use in other modes.
Freerunning allows players to evade incoming fire, especially in close-combat situations. It is a more offensive tactic in other game modes, as the set-up for non-Quarantine matches are different, and the enemy can be taken on much more easily than the Infected.