I don't know of this feature either. Maybe it's a reference to demos?

joshstrange · on June 21, 2013

Some of Valve's games will download the engine/binaries first then the levels after in order so you can start to play the game while it is still downloading. I have personally experienced this.

Also: http://forums.steampowered.com/forums/showthread.php?t=14004...

>>Apparently some of Valve's singleplayer games let you do so.

> Yeah, some do such as Half-Life 1 where it'll prioritize downloading of the binaries first, then the levels.

> Granted it makes for long load times if you hit a level transition for a map that's still downloading.

> Granted, the list that support that 'feature' are slim.