It can be quite hard to grasp what 8 Hours' Overtime for a Good Cause is. Even if you read or talk a lot about it. The event is different in every city and different every year. It's one of those things you should really just experience.
Still, it would be nice to give you at least some idea of what 8i could be like. Take a look at this video registration of '8 Uur Overwerken voor het Goede Doel' in Utrecht, The Netherlands in 2013:
And the video of the Harlem Shake wake up exercise at midnight :)
Both video's were made by Roald from Pyropix.