I have a 946 and have it set on the fast (normal) setting on the impellers. It can be set to do nothing to alfalfa or set very aggressively, or any setting in between. At first it will look like you aren't doing anything to the hay. But after a little while you will see the stems have the wax coating bruised off (from the crop rubbing against itself) and then you can gauge your setting for proper curing. In some varieties of alfalfa the cutterbar knocks off more leaves than the impellers do. You can play with the board setting and find what works best for you.
Most people that try impeller conditioning don't go back to rolls, at least in my area. Mike