In the usual case, the meter applies a known voltage and measures the current which, by Ohm’s law, gives you the resistance. It is also possible to control the current and measure the voltage ...

In fact, let’s say they add an ohm of resistance on each leg ... A conventional analog meter makes you zero out the probe resistance by shorting out the probes. But that requires some complexity ...

So, he built a single tool that combined multiple meter functions ... and then dividing the volts by the resistance (ohms).