Finding out the length of a cat6 cable
The meter uses TDR, Time-Domain Reflectometer tests to determine the length.


  CableIQ™ Residential Qualifier: Graphically maps wiring systems and shows distance to faults with built-in TDR


From wiki:


  A TDR measures reflections along a conductor. In order to measure those reflections, the TDR will transmit an incident signal onto the conductor and listen for its reflections. If the conductor is of a uniform impedance and is properly terminated, then there will be no reflections and the remaining incident signal will be absorbed at the far-end by the termination. Instead, if there are impedance variations, then some of the incident signal will be reflected back to the source. A TDR is similar in principle to radar.


Essentially, taking the know, to spec characteristics of ethernet cabling, it sends a pulse down the wire, and listens for the reflection that is sent back. Depending on how long the pulse takes, it's shape, and other factors, it can accurately estimate the cable length.