The back and major nerves:
I think its safe to say this was more of a scary bedtime story, maybe after some or a little too much mead, than real.
Not a bed time story ... the Vikings were really cruel they had to be less and they would not have been feared - then there is the continues modern Islamic practice Seen in ‘modern’ Syria of staking an individual down, opening the stomach, inserting burning coals and adding more fuel ... or castration of young boys using random piece of glass without anesthetic same for girls in the total removal of the labia by the same methods.
You need to pay a visit to a walk-through “Ripley’s Believe It Or Not” exposition on torture and the instruments used, and the results obtained by the showing - if you can find one in this day and age of dumb down to lowest common denominator, wherein, if it is not seen on TV, it never happened.
Recently, I told the true story of the Tiananmen Massacre: how the Chinese made 8000 plus people disappear from the official body count and the lady, hearing the tale, like the unbelieving historians, was horrified beyond words and refused to believe that it actually happened.