Funny that you're asking this now. Up until very recently, this was a huge secret that everyone was sworn to uphold for fear of facing the wrath of Kornel... But now that the source code for DRL has been released, things seem to be more relaxed about the secret, so...
You have to be berserk and have an empty equipment screen. (No worn armour or boots and no wielded weapons in hand or in the prepared slot) It's easiest to do if you have the berserker trait, but a Hatred Skull can help with making you berserk too.
...I have to admit that I feel weird talking about it now after all those years of secrecy, but there you go. Happy slaying! :)