I took the liberty of making the clip. In the clip shown i have Ill omen equipped and it shows that having cold steel on does not apply ANY slow stacks on heavy attack regardless of the animation. If it applied slow stacks the overload champion would have been stunned. I later use Ergo Sum to apply wolfpack rounds to the sword which then allows the wolfpack rounds to slow the overload champion causing it to stun. I do have the overload sword mod equipped and this also does not stun the champion in 1 heavy attack. It instead requires you to have overload blade buff in order for you to stun with the heavy attack which i assume is intended. Cold steel seems to work normally on light attacks. these are all the testing's i have found with this sword and perk.