Power Items pass the one IV from the parent to the baby. For example, if you wanted to pass down Emboar's Speed IV you would have it hold a Power Anklet.
The Destiny Knot will pass down 5 random IVs from BOTH parents.
Note that you cannot have one parent with a Power item and a Destiny Knot, or no IVs will be passed down. You can, however, have two different Power items.