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Under normal circumstances, thrust will be at idle. However, if a slow descent is required, or the speed has decayed then the thrust mode changes to speed. Similarly, if a speed increase is required (maybe due to ATC) then using a selected speed in excess of optimum will command the autothrust to speed mode, as the aircraft applies power to keep on profile.
If an increased rate of descent is required, OPEN DES must be selected and speedbrake used as appropriate. Selecting only speedbrake in DES mode will not achieve an increase in rate of descent, as power will be applied to maintain the airplane on profile and at target speed.
OPEN DESCENT
An alternative to the managed descent is the OPEN DES mode. This will give a descent with thrust at idle, and retains the option of using a managed speed or a pilot selected speed. Speedbrake is very effective in increasing descent rate. If the aircraft is not in managed navigation, i.e. in HDG mode, then managed descent mode is not available. The aircraft will not follow a descent path but will merely descend at an idle thrust setting, and a set speed. By monitoring the ND, the level off point can be found and the PROG page gives a deviation from planned profile. If in HDG mode, then the PROG page displays required miles to touchdown and actual miles.
A319/A320/A321 FLIGHT CREW TRAINING MANUAL  NORMAL OPERATION BRIEFINGS DESCENT - PROCEDURES AND TECHNIQUES - 1.02.17  Page 3 
REV 21  MAY 98 

RECOVERING IDEAL PROFILE
If the airplane is below profile in the OPEN DES case, the simplest way of regaining the profile is to use V/S. Once on the profile again, adjust rate of descent accordingly, with speed, V/S changes or OPEN DES.
If high on profile in the OPEN DES case, speed may be increased (with the permission of ATC), or drag used. Note that in DES mode, if drag is used when attempting to steepen profile, the airplane will apply power to offset the drag and maintain the ideal profile. Thus, to increase descent rate, OPEN DES must be selected and drag used, ensuring that thrust remains at idle.
EXPEDITE DESCENT (A320 only)
Another option is to use the EXPED pb which will command target speed close to MMO/VMO. This will give a very high rate of descent and its use must be closely monitored.
OVERVIEW
In all modes the ideal profile is tracked by the VDEV indicator on the PFD. It is important at all times to bear in mind terrain and MEA considerations. The procedure for terrain checking is especially important with thrust at idle. Bear in mind the drill for recovery from GPWS warnings QRH 1.08 (GPWS Warning).A rule of thumb calculation for descent is that track miles to run should equal three times your height in thousands of feet. Exact figures are given in QRH 4.10.
The effects of engine and wing anti-ice on descent profile can be marked, as the idle N1/EPR is increased, thus giving a shallower descent profile. If already in the descent, and anti-ice is used, it is usual to see an increase in speed in DES mode. If speed increases to the upper bracket, speedbrake can be used. If in OPEN DES mode a higher selected speed is advisable. Alternately, in OPEN DES mode, half speedbrake will counteract the effects of anti-ice. Conditions requiring the use of anti-ice are listed in FCOM 3.04.30 (Supplementary Techniques - Ice and Rain Protection).
It is vital to know the operating mode of the thrust system, particularly with regard to CFIT.
Weather radar use in the descent is covered in FCOM 3.03.17(SOPs - Descent).
06 - COMPLETION STANDARDS
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Uses the correct method to initiate descent.

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Monitors descent according to expected navigation.

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Maintains heading +/-5 and speed +/-10 kts.

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Adjusts descent rate using appropriate methods bearing in mind aircraft limitations and passenger comfort.

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Complies with all ATC clearances and minimum safe altitudes.
　
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