[Kanegasi]

When expiring buffs flash, Blizzard doesn't use any animation functions, they just change the alpha every frame. Here's the complete math behind it if you want to try matching it with animations or just use it:

function BuffFrame_OnUpdate(self, elapsed)
    if ( self.BuffFrameUpdateTime > 0 ) then
        self.BuffFrameUpdateTime = self.BuffFrameUpdateTime - elapsed
    else
        self.BuffFrameUpdateTime = self.BuffFrameUpdateTime + 0.2
    end
 
    self.BuffFrameFlashTime = self.BuffFrameFlashTime - elapsed
    if ( self.BuffFrameFlashTime < 0 ) then
        local overtime = -self.BuffFrameFlashTime
        if ( self.BuffFrameFlashState == 0 ) then
            self.BuffFrameFlashState = 1
            self.BuffFrameFlashTime = 0.75
        else
            self.BuffFrameFlashState = 0
            self.BuffFrameFlashTime = 0.75
        end
        if ( overtime < self.BuffFrameFlashTime ) then
            self.BuffFrameFlashTime = self.BuffFrameFlashTime - overtime
        end
    end
 
    if ( self.BuffFrameFlashState == 1 ) then
        self.BuffAlphaValue = (0.75 - self.BuffFrameFlashTime) / 0.75
    else
        self.BuffAlphaValue = self.BuffFrameFlashTime / 0.75
    end
    self.BuffAlphaValue = (self.BuffAlphaValue * (1 - 0.3)) + 0.3
end

Then they use AuraButton#:SetAlpha(BuffFrame.BuffAlphaValue) in the individual OnUpdate functions for each icon if timeLeft < 31 seconds, which means all expiring buffs pulse in sync with each other.