I've translated it below, but as always, please dig deeper and go take a look at the original psak in Hebrew.
Q: Is it permissible to practise yoga as part of a physical exercise involving the body and nefesh, without getting involved with the spiritual aspects of yoga?
A: If the practise has a foreign name, or it's somehow connected to the spiritual workings of man, or if it's a foreign way of life, you shouldn't get involved in it (all the more so parts of the shita actually go against halacha). Yoga is based on a pagan system. It's body movements mimic those of animals, and originates in Eastern idolatry. Therefore, this method is completely banned.
All the benefit that is felt from this practise is temporary, and ultimately also harmful. The benefit that is felt derives from the holiness of the Jewish soul. We need to educate ourselves to understand the difference between true, genuine holy light, and false replications of it.
Since the name of everything describes its spiritual origins, we are obligated to nullify / cancel the name of the foreign practice or system, as the first stage in its tikkun. After doing birur [i.e., seperating out the bad from the good], we need to change the name of the practise to something Jewish.
Therefore, it's completely forbidden to mention the name 'yoga'.
In order to come up with the correct Jewish name, a person needs to learn a lot of Torah and Chassidut. Everything has a short path and a long path, as explained in the Tanya. A person who tries to take the short-cut ends up taking a very long path, and he doesn't reach his destination, as is known.
All wisdom needs to be rooted in Torah.
Yoga has a negative energy that originates from tumah [spiritual impurity]. Therefore, it is forbidden, even if only for exercise and without any specific negative intentions [for idol worship]. All the benefits of this practise are like a shadow that distorts / covers / hides the true good that is available from movements (exercises] that are based on our holy tradition and rooted in the Torah's wisdom.
He's been researching alternative health subjects for many years, and really knows his stuff, both on the Jewish side of things, and also on the idolatrous spiritual practices side of things. At this stage, I would venture to say: CASE CLOSED on whether yoga can really be 'kosher' or not.