I have a Destiny Knot breeding question, in terms of how exactly it functions.
I have a Tentacruel with 4 perfect IVs, in Spe, SDef, Def, and HP. If I have this (male) Tentacruel hold the Destiny Knot, and a second (female) Tenatcool with a perfect SAtk holding the Power Lens, will this result in a Tenatcool with 5 perfect IVs (everything but Atk)? Could I replace the female Tentacool with a Ditto with perfect SAtk and get the same results? If the answer is no, could you explain what it is about the Destiny Knot mechanic I am not understanding? I ask rather than test because said Tentacruel is still trapped in my Gen 5 games, and I haven't been trading or farming for perfect IVs in XY of yet.
As a side note, I only sign into Smogon once every few months... Why is my avatar an animated Haxorous? I'm pretty sure I never put that there. Did I get randomly assigned that?
Dunno about the avatar.
Destiny Knot has the same effect no matter which parent it's attached to; it causes 5 IVs to be passed down, selected randomly from either parent, instead of 3.
Power items cause the linked IV to be passed down from the Pokemon that holds it.
The offspring will have the female Tentacool's Special Attack IV, guaranteed. They will have four other IVs passed down from either the Tentacruel or the Tentacool, each one selected randomly. They will have one IV rolled randomly. (Yes, the guaranteed IV from the Tentacool does count against one of the five IVs passed from the parents by the Destiny Knot!)
(As a side note, you ought to evolve that Tentacool before you try breeding; if the pokemon species matches, it significantly boosts the egg production rate, and with matched species AND the Tentacruel having a different OT, they'll be popping out eggs faster than you can hatch them.)
Replacing the female Tentacool with a Ditto with the same x/x/x/31/x/x spread and a Power Lens would have the exact same effect, so stick with the Tentacool and evolve it for a higher egg rate.
To summarize briefly, by itself, for simplicity: Destiny Knot, when held by either parent in Gen 6, causes all offspring to inherit 5 of their 6 IVs from their parents, instead of the normal 3. All 5 IVs are still randomly selected from the two parents, so who holds the destiny knot doesn't matter at all.