The tabs are bent in a maintenance operation, using a tool that fits to the blade and that allows precise measurement of the tab relative to the blade's chord axis. The tool usually used a dial indicator that can read .001" differences. Maintenance instructions describe how to bend them and when to do so, based on the vibration readings from the appropriate vibration measurement gear.
Interestingly, the blade flies counter to what intuition would have you believe. A down tab movement would seem to be like an elevator that would make the blade rise, but in fact, the blade is a noodle, and a down tab makes the blade twist downward, so it flies downward.
For an eye-opening video of blade motions that support the noodle theory, search on youtube for "rotor.avi tom seagrave" to see an S-56 rotor blade in flight.