Lord Spandex:
Essentially FLCH will give you flight idle in the descent until the AP
captures the selected altitude and the AT will then revert to speed mode. If,
for whatever reason, you turn off the AP and try to level off above the selected altitude the throttles will stay at idle and, therefore, your speed will continue to reduce.
If one turns off the AP, would not one then begin to move/adjust the power levers manually as a matter of habit/training?
A couple of the posters who commented on the FLCH trap feature mentioned other modes. For my own understanding, getting a better look at the system may help.