As already stated, it is necessary to start the process by setting and browning the outside of the meat sufficiently, in keeping with its size; the reason for this being to drive the juices to the centre of the joint to prevent them from escaping and to build up a coating on the outside. This concentration of juices increases from the outside to the centre in keeping with the progression of the cooking.
Under the influence of the heat of the surrounding liquid the fibres of the meat contract, so forcing the juices they contain towards the centre of the joint. When the full force of the heat reaches the middle of the joint the juices are fully concentrated and drive out the water content which is vaporized; it expands causing the breakdown of the fibres of the meat.