How does abeMTdebug work?

I'm trying to debug some ABE code that is going awry, and can't work out how abeMTdebug is supposed to work.

The documentation says: "Enables the output of additional messages to monitor progress when multithreading is used" but doesn't explain where those additional messages go. There do not seem to be any additional messages in CDS.log or the CIW, and running the code from within the SKILL IDE doesn't seem to result in any extra information being displayed.

This snippet of code is representative of what I'm doing:

(setq obstruction_layer (abeNewLayer))
(abeMTdebug obstruction_layer)
(foreach layer sizing_queue
  (unless (zerop (get layer "numTiles"))
    (abeLayerOr layer obstruction_layer ?queue t)))
(abeRunQueue)

sizing_queue is a list of ABE layers (the result of abeLayerSize). It is created by cons'ing layers onto sizing_queue as they're created (I have confirmed sizing_queue is a valid list of ABE layers). Layers are only added to sizing_queue if their numTiles is non-zero (ie. they all have some shapes in them - the check of numTiles in the code snippet isn't actually necessary). I do confirm that abeMTdebug returns t. I'm running virtuoso from ICADVM20.1-64b.500.29.

Any hints on how I should be making use of abeMTdebug would be greatly appreciated, as would any other hints on how to debug multithreaded ABE code.

thanks.