Watchtower has historically interpreted Heb 4:12 as emphasizing the power of the holy scriptures to bring about changes in an individuals heart and actions. Most Christian faiths take the "Word of God" to mean Jesus and his teachings.
Now in the Sept 2016 study WT we have new light! Its so clear now! According to WT the Word of God now means "God's expressed purpose" as contained in Bible - of course as interpreted by the Faithful and Discreet Slave!
When I read this it struck me as a rather significant change. It shows the WT doubling down on their claims to exclusive knowledge of God's purpose and a further denigration of Jesus and the Bible itself. According the this new understanding, supporting the Watchtower means the word of God is exerting power in your life. Rather than letting the word of God mold you as a person they are now saying that obeying the directives of the GB is what makes you a better person and brings about God's approval.
Probably someone else can word this better, but I think this new understanding is quite extreme to me.