wb2002 wrote on Jan 28^th, 2012 at 6:28am:

On landings I don't use speedbrakes to slow plane - I use them (now called spoilers) to destroy wing lift after touchdown. I use speedbrakes sometimes while trying to descend quickly and not pick up speed while doing it.

There is an airline and/or FAA thing which sets a minimum altitude for speedbrake mid-air deploy. You shouldn't use them below a few thousand feet from the ground, and that's pushing it. Your speed may drop close to stall, specially if you have already selected a flap setting.

Look, there is a maneuver for rapid descending spin-push-spin-pull, if you have a bit trouble going down at time and holding your desired speed. It's designed to descend quickly and smoothly without 360ºs or agressive dives.
Terrain may be higher than 9,000 ft and your autopilot with autothrottle must be properly engaged to this procedure.
-Spin: spin the MCP altitude to your setting alt.
-Push: push FL CH.
-Spin: spin the airspeed up to above Mmo(Mach maximum operating)/Vmo(Maximum operating speed). Initially spin it above and then adjust it to less once the descent is established. Do not overspeed.
-Pull: pull the speedbrake lever.

From the FL300+ club, predict your dive and keep and eye on the speed so you may have full control when you begin approaching your destination.

Rigo.