gistfile1.lua

local driver = CreateFrame("Button", "casthack_driver", nil, "SecureHandlerClickTemplate")
local executor = CreateFrame("Button", "casthack_executor", nil, "SecureActionButtonTemplate")

driver:SetAttribute("_onclick", [[
    local executor = self:GetFrameRef("executor")

    if math.random() < 0.5 then
       executor:SetAttribute("type", "spell")
       executor:SetAttribute("spell", "Concussive Shot")
    else
       executor:SetAttribute("type", "spell")
       executor:SetAttribute("spell", "Arcane Shot")
    end
]])

driver:RegisterForClicks("AnyDown")
driver:SetFrameRef("executor", executor)