I think the word EXPEDITE quite clearly conveys the required message.
Why go to the trouble of imposing a specific climb/descent rate, unless you're doing a fancy climb under/descent on top?
"Descend immediately" - "ok" thinks pilot, "down I go immediately - at 500fpm"
"Increase your rate of descent" - from 500fpm to 510fpm????
"Expedite your descent through FLxxx" - "I require you to pass the level stated as soon as you possibly can!"
Obviously NOT TO ENSURE SEPARATION. But to get over/under another sector maybe - or in order to release on own navigation sooner? There are plenty of reasons.
Oh, and it's also STANDARD PHRASEOLOGY.
I personally would and do use it. What you do is up to you.
[ 27 July 2001: Message edited by: Goldfish Watcher ]