This feature is working. It will display SQL statements in case of specific errors. As long as you don't have the error, debug mode won't show anything.
It also shows some other things.
Just scan the code for iforum_message. This is the function which is used to display the messages.

Updating tickets (#649, #771, #787, #788, #789, #790, #791, #792, #813, #814, #815)

Updating tickets (#23, #298, #28, #278, #315, #317, #325, #326, #27, #186, #220, #254, #311, #312, #313, #314, #316, #319, #327, #24, #25, #26, #29, #30, #188, #189, #230, #236)