I tried this technique about one year ago. I had a large Tulip Poplar stump that I had roughed into a 21" blank and had read an article about this process. I tend to disagree with the tyvek concept though as the idea is to slow the drying process, not hold the water in entirely. The paper bag allows for a non water-tight barrier for the moisture to travel though.
I didn't have any big hurry to get the blank finished, so I bypassed weighing it and all that mess to determine the water content and just kept it bagged with chips for 3 months, then again without chips for another 3 months. After that I tore a small hole in the bag and let it sit until I pulled it out 6 months later and turned it.
The blank had not cracked at all even though it showed the warping associated with significant drying. Although it took a while it was well worth the effort. Since that time I've taken care to turn circular blanks (what I call "Cheese Wheels") ahead of time and seal the ends. Then after turning my bowl blanks I use the bag method. It works really good.