Offpath Descent - Energy Management Rings
Where fitted (think only on pegasus/PIP FMC's) the Offpath Des when selected on displays 2 rings around the aircraft, one is a 'clean' ring the other is a 'drag req'd' ring. If you are outside the first ring (idle power) you may find you are in a low energy situation and may need to use power. If you start getting inside the second ring - drag req'd. You may find yourslef getting hot/high etc etc. As long as you remain inbetween the two rings you are in a good position to fly an idle power approach
Its quite useful initially I found when training but after a while become a bit distracting.