Imagine sitting at point A, early on the STAR. Whoops, shortcut by ATC, all of a sudden you are high.
Assume you just get as draggy as possible, reduce to idle and then plod along without increasing speed significantly. You will end up at point B at a certain altitude dictated by your L/D ratio at that airspeed.
Imagine instead that you get as draggy as possible, reduce to idle and increase the speed as much as you are allowed by pointing the nose down. At the altitude of point B you level off and fly to point B.
The name of the game is to get rid of energy. Which method will have you slowest by point B? That method is the most effective in getting rid of energy in the number of track miles between A and B.
With the second method, where you increased speed, you will fly a longer distance A to B and at a higher speed, meaning increased drag compared to just flying the straight path (the backside of the power curve is not likely to be seen in that phase of flight).
Energy loss equals drag times distance covered. You will have less energy (i e lower airspeed) at point B if you pitched down to increase speed. If you instead only levelled off to the initial airspeed and then descended at this airspeed, you'd be lower at point B using the second method.
Every glider pilot is well aware of this. If you are way high, pop the boards and dive for the ground.