While I was at a workshop at Chase Rosade's on Wednesday, another fellow had a huge procumbens cascade that had belonged to an early master of American Bonsai. It had literally exploded its pot! (A real shame, as it was a fine Jim Barret pot worth hundreds of dollars - it probably wiehed 50 pounds!)
As he was starting to repot it, Chase stopped him before he had finished putting the soil in, and said, "Put the tree in now!" He then went on to point out that too many cascades wind up with all of their roots going round and round in the top of the pot.
This can be prevented by putting the tree in the pot, and then adding soil to bring it up to the desired level, literally lifting the tree as more soil is added. (Most of us probably need an extra hand to do this!) Doing this avoids compacting the roots under the tree.
Think of how we pot in a regular pot. We build a mound of soil and twist the tree into it to fully engage the roots with the soil and to stabilize the tree in the pot.
With the added depth of the cascade pot, there is room to let the roots dangle within the soil, which can only benefit the tree. That is what is accomplished by potting this way.
Another little thing I would never have thought of.
Sometimes the most valuable thing you can do at a workshop is watch!