I saw it demoed at MacWorld (January 1996 I presume), and they did the following demo:
1. They spun up a bunch of apps: played video, rendered 3D, etc. The BeBox had two sets of LEDs on the front that reflected CPU usage, so you could see the load increase.
2. They would keep loading apps until both meters were permanently pegged: 100% CPU usage on both. This was something like twenty windows, mixed across a bunch of tasks.
3. They would grab a single window -- say the 3D render -- and drag it rapidly around the screen. You would see the 3D render immediately speed up as it came to the foreground, and all other tasks slow down noticeably as the Be rep dragged the window around the screen. The window moved smoothly.
4. Then, they would disable one processor, leaving one to support the load that had sat two at 100%. Everything on screen became very slow, except the foreground 3D render, which still moved with reasonable speed.
5. They would again grab a window -- maybe a video this time -- and drag it rapidly around the screen. Every other window slowed to a crawl (but didn't stop). The video window moved around the screen smoothly, and the video kept playing smoothly as well.
All this to say that BeOS knew who it served: the user with their hand on the mouse. You clicked a new window, that window immediately received enough CPU to be responsive. Give the OS a loony task, and it would still do its best to prioritize whatever you told it was important.
I have never seen an OS before or since that handled multi-tasking as well.