daft parameters

His heels come worrying close to the back of his head sometimes when he does the running-into-the-sofa-and-flipping-his-legs-up thing. Presumably he'd stop if it hurt but presumably it hurts when he hits himself on the head with bits of train track yet he doesn't always stop immediately. The closest I've ever been to kicking the back of my own head was on the second time I went snowboarding when I faceplanted but felt my legs continuing forwards as my chin slid to a stop. It wasn't a particularly pleasant sensation but perhaps it's easier to pull off without needing to check that you can still move afterwards when your bones are still slightly bendy.