Shows transparent icons for AoE taunt and Intimidating Shout on Vashj after phase 2 (add phase) has started. The idea being that you get everything on CD before phase 3 starts.
Not sure if there would be a reliable, better, later trigger to use before phase 3 starts, like checking Vashj's health or buffs for one dunk remaining. Not sure that's possible in classic without relying on the player targeting or mousing over Vashj.