The ascender and prussic knot work exactly the same way and do the exact same thing. The ascender just does it alot better and allows for easy one hand adjustment in both directions.
Both require you to release mention on the rope to allow it to move exactly the same way.
The ascender is the best system out there so far. With the amount of stands I hang and take down a season if there was a better system I'd be using it. And if a better system ever comes out I will be first in line to get it. Until then the ascender is only thing I will use.
Lt-m-grow, I think you are thinking of a life line not a lineman belt. With a lineman belt you just inch worm your way to the lower step or down the tree. With a lifeline the physics are the exact same with a knot or ascender. You have to release tension on each to get down or to the stick, step, etc that you can reach. You just grab the main line with one hand and release tension on the ascender or knot with other hand and slide down a bit the repeat. Both work the same. But the ascender does it all alot better.