"In practicing yoga, he would assume poses extremely abruptly, whereas one needs to enter asana very slowly."

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Note added at 25 mins (2010-02-19 16:40:50 GMT)
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I don't like the parallel construction and side-by-side adverbs, so perhaps you could reword it:

"He was extremely abrupt in assuming poses when he practiced yoga, whereas the correct way is to enter asana very slowly."

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Note added at 1 hr (2010-02-19 17:27:20 GMT)
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First, a correction: One talks about entering "an asana" - you need the article, when using this non-English word in English. And "poses" can also be "postures."

You can say "entry into an asana," if you have this as a recurrent phrase.

"He was extremely abrupt in assuming poses when he practiced yoga, whereas entry into an asana is supposed to be very slow."

If you feel that you absolutely need to use a noun in the first part of the sentence, too, there are various options:

"His entry into yoga poses was extremely abrupt, whereas entry into an asana is supposed to be slow." (Again, that's a more boring way of writing; I would stick with a verbal construction in the first half.)

"The way he assumed yoga poses was extremely abrupt, whereas entry into an asana should be very slow."

"He had an extremely abrupt way of assuming yoga poses, whereas entry into an asana should be very slow."