CAUTIOUSLY BUFFING Sejuani. We're also removing some elements to her passive that feel unintuitive for most players and aren't capitalized on. If we proactively remove it now, we'll have more room to buff her in the future since pro players won't exploit the safety it brings.
Her E stacks on medium, large, epic monsters, and champions when SHE and nearby allies basic attack them. The attack speed allows her to freeze and deal more damage to targets faster in the trade. In theory she would shed damage earlier in the fight, but she would be less healthy as a result of the fight because she has less resistances and she would still need to wait for the cooldown before she could apply additional stacks.
I would take the "CAUTIOUSLY BUFFING" to mean it might not make it live.