Sure thing A mod-shift binding is a type of binding you can apply to a keyboard key or mouse click (and maybe a joystick button) which modifies the action if the shift key is held down. In my case, it is shift-clicking a switch to operate it the other way from a normal (not shifted) click.
Basically, it doesn't seem to work the way the FG documentation says it does.
The shift key itself does not generate a keystroke code on its own but maybe you could use this (though it seems a bit sluggish to change over)
var shift = props.globals.getNode("/devices/status/keyboard/shift");