I know it can be perfomed on some 3560/3750, 4500 and 6500 modules, but what is it?
TDR is Time Domain Reflectometry and is used to determine where a break or a bend in a cable is.
The TDR stand for Time Domain Reflectometry. The basic idea is to emit a pulse onto a medium - copper or wire - and expect a reflection from the other end or from any significant discontinuity in the medium, such as break, splice, connector and so on. From the knowledge of the time interval between the pulse and its reflection and by accounting for the speed of signal propagation on a given medium, the distance of that heterogenity from the measurement device can be calculated.
On these switches, the TDR measurement can be used to assess the length of a cable connected to a particular port. If the cable is broken, the TDR will help you estimate the approximate distance of the break. It is very helpful when, for example, only one or two wires from the entire TP cable are broken while other are undamaged - in this case, you will see that the length of a particular pair is shorter than other pairs.
Especially useful is the OTDR - Optical TDR, to determine quality of fiber installation and for determining possible problems.
You can use the TDR feature to diagnose and resolve cabling problems. TDR is supported only on copper Ethernet 10/100/1000 ports. It is not supported on 10/100 ports or small form-factor pluggable (SFP) module ports. For more information about TDR, refer to the software configuration guide for this release.
After you run TDR by using the test cable-diagnostics tdr interface interface-id command, use the show cable-diagnostics tdr interface interface-id privileged EXEC command to display the results.