Detecting a parasitic voltage/current that differs from line frequency
If a large motor with inertia is running at 50 Hz and grid hypothetically shifted 0.5Hz the current would change within 2 seconds or 1/f and might last longer as it reaches steady state.
This is why the grid only changes slowly to ensure load does not exceed supply by lowering f. Then return to normal or higher slightly in anticipation of next expected wave of load power.