The sacrum is a hard nut which we endeavour to crack open with the work, if only to discover why on earth it's called the sacred bone. The weight of the upper body (gravity) drives the sacrum down into the jaws of the pelvis so that the legs – the handles of our nut-cracker – can apply a pumping pressure. The function of the mind is to remain still and open so that this precarious structure maintains its alignment, and to focus in on the job at hand – to remain mindful of what it is trying to achieve. The mind needs an image, a concept, an idea of what's happening, otherwise it becomes anxious or bored. But such images are only ever pacifiers, and we must always be ready to drop them and find new ones when they stop working.