Think about the reverse, a level off. You climb at say 110kts but want to end up at 140kts. As you approach the height you require (QFE set !!!) you set the att required for initially 110kt cruise (as that is the speed you are at) and then as you accelerate you need to lower the nose (and TRIM) as the att required for the 140kt cruise will normally be lower than a 110kt cruise.
The situation described is essentially the same in reverse. There is no mention as to why they are in a glide descent and the use of full power would indicate to me that they are not paying for the fuel (military training?) however the technique is correct and works on many different types, JP5, Hawk, Grob, PC7, Tucano, CT-4E in my experience !