It is indeed a scriptfile using the engine to render the demo/scene you are seeing. A video would take up a lot of hard disk space. Think of it like a demo akin to timedemo mechanics implemented into other games like Quake, Doom, etc. Enemy behavior is not sporadic, but hardcoded into the scriptfile to guarantee an exact replication of the render each time to make benchmark comparisons feasible. (Which is not the main purpose of the Descent demos, though.)

You can record demos yourself by hitting the appropriate hotkey whilst in game to start/stop recording. Afterwards the demo will appear in the demo list in the main menu. The scriptfile will record all movement of entities, projectiles, etc., to be able to reconstruct it afterwards with the original game assets. Effectively, you are looking a a "new" rendition each time you're watching the demo, albeit a 100% identically reproduced one, like playing the exact same single player round with the exact outcome each time.

Well, since it's a script running the normal game(except scripted instead of relying on direct input), since it's not a video playing, it makes it ideal for testing and optimization emulators, doesn't it? That's what I've been using it for mainly ever since my emulator could run that without visible problems.

It's not the main focus, but it is still one of them. One of the shipped demos is a rather demanding one for this very reason. (The one with the lava hub, forgot the level number)

superfury wrote:

it makes it ideal for testing and optimization emulators, doesn't it?

One common complaint with such scripts/timedemos is that everything is pre-recorded, so part of the engine is bypassed (no user input, no AI, no collision detection etc). It is mainly a test of the rendering backend.