Turing on the camera AFTER the taser hits only lets us (the cop-cam viewers) see if the cop then refuses to call in the subject's distress when a pacemaker goes on the haywire or if the subject suffers an epileptic fit.
The point of the body cam is to record the actions before the use of force, to determine whether on not excessive force was used, and secondarily whether alternate tactics would have had a better outcome. At a minimum, turn on the camera when the officer exits the vehicle so as to get context; if necessary, the camera can erase non-force exits after vehicle re-entry to prevent using up all the recording time.
Of course, my algorithm fails for foot patrol officers. Perhaps for them, record minute long segments and discard and record over any too long before a triggering event like the taser discharge, then convert the (timestamped, of course) segments into a continuous record as needed. This requires a bit of controlling intelligence on either the body cam or a worn pack, but shouldn't require too heavy a controlling unit, I should think.