http://www.watchtower.org/e/20020801a/article_01.htm
Here is a 2002 article on their website. It reflects past and current official WTS policy on yoga.
Not a Choice for Christians
Whatever health instructors may say to the contrary, yoga does not stop with physical exercises. The book Hindu Manners, Customs and Ceremonies relates the experiences of two yoga novitiates who were under the guidance of a guru. One is quoted as saying: "I made superhuman efforts to hold my breath as long as possible, and only breathed when I was on the point of fainting. . . . One day, at high noon, I thought I saw a bright moon, which seemed to move and sway from side to side. Another time I imagined myself enveloped in thick darkness at midday. My director . . . was greatly pleased when I mentioned these visions to him. . . . The time was not far distant, he assured me, when I should experience much more surprising results from my penance." The second man relates: "He obliged me to stare at the sky every day without blinking my eyes or changing my position. . . . Sometimes I thought I saw sparks of fire in the air; at others I seemed to see fiery globes and other meteors. My teacher was much pleased with the success of my efforts."
The strange sights were evidently what the gurus felt were proper results along the way to the true aim of yogic exercises. Yes, the ultimate goal of yoga is moksha, explained as the merging with some impersonal great spirit. It is described as "the (intentional) stopping of the spontaneous activity of the mind stuff." This is clearly contrary to the goal set out for Christians, who are given the admonition: "Present your bodies a sacrifice living, holy, acceptable to God, a sacred service with your power of reason. And quit being fashioned after this system of things, but be transformed by making your mind over, that you may prove to yourselves the good and acceptable and perfect will of God."—Romans 12:1, 2.