Destiny Knot guarantees that 5 IV's are passed down. The Power item means that one is guaranteed for that stat.Quick and slightly complicated question about IV breeding.
Note that nature is irrelevant in this case.
If I have 2 perfect IV parents, and one is holding a destiny knot and the other is holding a power item, do I have a one in 6/12 (I'm bad at this kind of math stuff) chance of getting a pokemon with perfect ivs or a one in 31 chance of getting a pokemon with perfect ivs due to the power item automatically choosing one of the 5 IVs that's passed down via destiny knot?
You end up with the same chance of having a 6iv compared to a 5iv as always, youre just guaranteeing that your power item IV is being passed.