On the assumption that the speaker was not standing with a handful of small metal nails in order to do some DIY, we can only assume that it was his fingernails which sank into his own flesh, with the seriousness of the oath he was taking.
Originally Posted by unpakwon
Look at the definition of "mind over matter" here. When we say that "there is something to something" we mean "if there is any truth in it".
So basically, they are saying that if it is true that a person can use their willpower to overcome adversity or something unpleasant, then the speaker will be successful in winning and surviving.
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