It is a series resistor inside capacitor, using 100kHz to remove the impedance 1/(2*pi*F*C). The impedance become small, and we can then measure the true series resistor value. Using this theory, we can measure the capacitor is bad/ damaged or good in condition. Our ESR meter only applies less than 15mV DC or peak to peak on a good capacitor, as a result we can use it as in circuit test. During repairing TV, LCD, Audio board, etc. we can in circuit testing the capacitor is good or not. Dual terminal, for fast and easy inspect general capacitor or resistor, a printed ESR table for fast check.