Operation of a Growler to test armature shorts
I've worked for the last 20 years with DC motors - but have never had the need to use a growler.  My understanding it is used to check for shorted commentator segments.  The resistance of armature circuits is normally very low so it can be difficult to see a shorted segment.

The best explanation I have found is a Youtube video:

https://www.youtube.com/watch?v=yXQ0heuV6VE