Being dead has some interesting effects like being unable to punch or sprint, and collision is very wonky. It enables a decently-sized zip in Bonehell. Probably useful somewhere else too, haven't bothered to check. At any time during being 'dead' you can press start to instantly revert to a checkpoint. All you need to do to start a level dead is hold ESC on the main menu until you die, then hit continue.

To instakill house, just press ctrl, that's all.