'Professor Snape's very interested in the Dark Arts,' he blurted out.
'Really?' said Lupin, looking only mildly interested as he took another gulp of potion. 'Some people reckon —'
Harry hesitated, then plunged recklessly on, 'some people reckon he'd do anything to get the Defence Against the Dark Arts job.'
After researches in dictionaries, I found this sense of 'plunge' might fit for the context:
6. (intr) informal to speculate or gamble recklessly, for high stakes, etc
But I haven't found a reference anywhere about the phrase "plunge on". Can we just remove 'on' from the context: Harry hesitated, then plunged recklessly, ...
What does it mean exactly in this context?