The TRE is really just a resistor, that alters the gear position sensor signal to make the ECM (Engine computer) think that your in 5th gear all the time. What's the advantage? well...normally each gear has it's own fuel map and ignition setting that the ECM tells the engine to run at, from the factory 1st through 3rd gears have a "map" that provides for a little better emissions and also has a diffrent ignition setting that unfortunatly can create a small hesitation when driving normal and under 4500rpm's....by using the 5th gear "map", in most cases, 1st through 3rd can be smoothed out and be more responsive to throttle demand up to 4500rpm (better Snap)... Also because the the speed restrictor is programed into the 6th gear "map" this should eliminate the speed restrictor because the ECM never see's the 6th gear Map (your always in 5th).
Because your engine always "thinks" it's in 5th gear from the TRE, it also never knows when your in neutral, so this mod will affect your Idle speed, this is one of the reasons why some guys will have it wired through a switch so they can turn it off when the want.