This is one of the frequency measureing methods that was thought out to make up the defect of direct counting method, and it is prevailing now. This method measures the period (T) of the input signal instead of number of cycles, and calcurate its frequency in the formula f = 1 / T with a microprocessor. Its frequency resolution depends on only time resolution independent of the input frequency.
It can measure low frequency with high resolution quickly however any trigger gitter due to noise decrease measureing resolution. To maintain sufficient time resolution, it generally measures the period of several number of cycles. The reciprocal method seems to also be impmemented to cheap frequency counter because of price of microcontrollers falls.