The xscreensaver man page says:

POWER MANAGEMENT
       Modern X servers contain support to power down the monitor
       after  an  idle  period.  If the monitor has powered down,
       then xscreensaver will notice this, and will not waste CPU
       by  drawing  graphics demos on a black screen.  An attempt
       will also be made to explicitly power the monitor back  up
       as soon as user activity is detected.

It lies. It still runs while X has put the monitor into standby. Taking an
xwd confirms that the module really is displaying - it's not just that the
check for DPMS is done in the submodules rather than the xscreensaver
process.

passion /home/dwmw2 $ ps axf | (sleep 1 ; grep --after-context=1
xscreensaver)
 1311 ?        S      0:01 xscreensaver -no-splash -timeout 10 -nice 10
 3404 ?        SN     0:02  \_ penetrate -root
passion /home/dwmw2 $ xset q | grep --after-context=1 DPMS
DPMS (Energy Star):
  Standby: 0    Suspend: 0    Off: 1200
  DPMS is Enabled
  Monitor is Off
passion /home/dwmw2 $ 


On a desktop machine, this is harmless enough - aside from making me think
X had crashed because it was taking 99% CPU. On a laptop where we actually
want to conserve power, it's more important.

It shouldn't matter, but the X server is somewhere between 4.0.1e and
4.0.1f on SMP x86.