1. Head to the particle accelerator room (Next to the shield/weapon upgrade)
2. Jump on the railing going up towards the computer stations
3. Line yourself up with the middle of the railing post and the pipe sticking out of the machine on the level below you
4. If done correctly you should notice that all zombies will now be standing in place (frozen)
5. Zombies will not attack you, and it even works on the boss!