Normal airflow over a symmetrical tail boom will generate (via the Coanda effect) an equal high pressure flow along both sides of the boom.
By installing a strake on one side of the tail boom, the airflow is disrupted, creating a low pressure airflow on that side of the tail boom. With the undisturbed (high pressure) flow on the other side, the boom then takes on the attributes of an aerofoil, creating lift toward the side with the strake, thus offloading the anti torque demand on the tail rotor, and effectively increasing tail rotor authority. Most efficient in the hover, with the full downwash of the main rotor flowing over the tail boom.
Various after market kits available, plus the A109E Power and the AS350B2 have strakes as standard.