problem starting/stopping auto tracking

[hotspur1]

The documentation says that I can start and stop the automatic tracking using the following notation

viz.tracker(viz.START) or viz.tracker(viz.STOP)

Unfortunately for me, this doesn't work.
I have an IS-600 and am using the intersense.dls plug-in provided with Vizard version 2.12 (patched version for the callback problem). I initialize the tracker as usual with

trk = viz.add('intersense.dls')

Does this feature work yet? Is there a workaround? I would like to do an experiment comaring head tracking on or off, so to be able to switch on the fly would be most useful.