Valve has detailed the free Survival Pack update for Left 4 Dead, claiming that "the number of zombies killed in a single round often outnumbers an entire campaign".
In a post on the Left 4 Dead blog (thanks Kotaku), Valve's Kerry Davis and Scott Dalton explain that the idea is straightforward: to last for as long as possible while the game heaps a neverending rush of Infected upon your team in waves.
Games should "regularly last under ten minutes, and emphasise competition with team play through leaderboards", the update says.
However, this being Valve, it also goes into detail about how the mode was developed, with particular attention afforded to the pacing, which had to deal with variables like boss-class Infected, player skill and gameplay lulls to engineer something competitive that doesn't "cross the line from having fun to just getting worn out".
There's even a graph showing off how the team tried to "visualise these overlap points and the collapse of each spawn curve".
As you probably remember, the Survival Pack will be a free update for PC and Xbox 360 owners, and is due out on 21st April. As well as Survival Mode, it opens up the Dead Air and Death Toll campaigns for Versus mode.