Not true. It is called the autoblock for a reason. You tie it by wrapping it around your rope. To descend, you simply apply a slight amount of downward press

Not true. It is called the autoblock for a reason. You tie it by wrapping it around your rope. To descend, you simply apply a slight amount of downward pressure to the top of the knot. This is the easy to release part. If you remove that pressure, by just moving your hand off the knot, it instantly grabs and stops all downward movement. A Prussik will retain most or all of it's tension, making it difficult to use in ascending or descending. I only use a Prussik in a situation where I want no movement at all for the duration, such as a backup to an anchor.