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Rudi,
NanoSaver uses FFT methods to compute what would happen if you sent an impulse (sharp voltage spike) down the attached line. One trace represents the computed reflection of that impulse from irregularities on the line, like poor connections or changes if line impedance (say, 50 ohms to 75 ohms). The other trace attempts to show the impedance of the line as a function of distance. Unfortunately, the impedance trace does not quite work properly yet: it always shows increasing impedance. A 50 ohm line follwed by a 75 ohm line will show the expected step up in impedance, but a 75 ohm line followed by a 50 ohm line does not show a step down, instead it shows a step up. The developers are working on correcting this.
The firmware on the NanoVNA itself does this correctly. See my message 12494 on how to set up the NanoVNA to show the reflection of a step wave into the cable and read out the impedance along the line.

--John Gord