Open up the Script Editor, and turn on History > Echo All Commands
Also open the Window > General Editors > Attribute Spreadsheet.
From the All tab, find Keep Map Borders.
As you change the radio button between Do not smooth, Smooth internal, and Smooth all, you'll see the value in the Attribute Spreadsheet change between All, Internal, and None.
Type All in the Attribute Spreadsheet, and look over at the Script Editor. You should see this line:
setAttr "pCubeShape1.keepMapBorders" 2;
That is the MEL command that sets Map Borders to Do not smooth