[plt-scheme] How exactly does one debug programs with DrScheme?

I'm using DrScheme 351, with the module language.  I see a "Debug"
button, which brings up a "Pause", a "Continue", and a "Step" button.
But it's not clear what those buttons do, nor is it clear if I can set
breakpoints.  I couldn't find anything in Help desk by searching for
the words "debug" or "debugging" {searching for "debug" yields a few
hits like drscheme:debug:add-prefs-panel, which doesn't seem
pertinent; searching for "debugging" yields no hits at all}.

Is the debugger documented anywhere?
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