the dead spot refers to the spot where you can move the steering back and forth without moving the tires. I have a pretty good one in my steering, enough that I'm not driving my rig on the street again until I replace the spacer, it wanders all over.. mine is worn pretty badly though.
the TRE at the passenger knuckle will roll over forward and back a good amount before the knuckle itself starts to move without the spacer, the spacer helps but as weirdo mentioned it will still be there. it goes around the shaft of the TRE and takes up the space between the face of the TRE and the knuckle so the TRE can't pitch over as easily.