Hi guys,
'Iron heel' certainly refers to tyrannical rule.
However, I wouldn't say it refers to a torture device used to crush one's heel. The usual 'image' is that of a tyrant wearing iron heels and 'stepping on' his opponents.
Consider this quote from http://london.sonoma.edu/Writings/IronHeel/chapter9.html
Then,' Ernest answered, `you, and labor, and all of us, will be crushed under the iron heel of a despotism as relentless and terrible as any despotism that has blackened the pages of the history of man. That will be a good name for that despotism, the Iron Heel.'4
Best wishes, Clive