Kayn's transformation is gated behind collecting souls (orbs) that drop when he deals any damage to an enemy champion.
AS OF LIVE, this includes the Damage Over Time from Deathfire Touch and Corrupting potion, dropping 1 soul per tick of damage.
This effectively means a Kayn player can hit an enemy champion once with corrupting potion up, and get 2-4 times the number of souls he otherwise would have.
This in turn means he unlocks his transformation earlier, and as such, his full potential.
Riot recently became aware of this (likely due to this being used to great effect in the EU LCS), and have decided to remove the interaction, as it is in fact unintended, and is a bug, rather than a feature.
This is lame cos it gives Lane-Kayn the dick.