The iron maiden is a fictional torture device with no real historical usage, as far as I know.
However, the presumed basis of it as a torture device is that once you were enclosed you would be completely unable to move without being stabbed, cut, scraped, etc., by the needles therein.
Salem said: Isn't that an execution device more than torture device ? BAM, you're dead
Depending on the source the spikes are either just distant enough for a person to be stuck in there without moving (albeit stabbed whenever they make the slightest movement) or as an execution device the spikes are close enough that they do indeed stick a person regardless.
The former is more prevalent than the latter, however.