Centres are connected in a particular way and energies are distributed between them in a certain specific way. When people are lazy, centres try to do one another's work and use one another's energy. Physical work is a reliable method of making them work better. Hard physical work (not sport) makes each centre use its own energy correctly.
In the Work, physical work is often undertaken as second line work, i.e. within the school and for the members of the school. When a certain number of people work together---in a house, in a garden, on a farm---it is not easy. They can work as individuals, but working as a group is difficult.
Suppose you work in your own garden. You will do the things you like to do and do them in your own way. You will choose your own tools, your own timetable, your own weather. So you introduce self-will into the situation. In organised work within a group you will struggle with self-will.
Self-will always knows better and always wants to do things in it's own way. From a school perspective self-will spoils the whole situation not only for the individual but also for other people.
Organised physical work needs emotional effort. If there is no emotional effort in the physical work being done then either the speed must be increased, or the effort increased, or the time reduced in order to make it emotional.
If you do physical work harder and longer than you can do it with ease then you will see it requires an emotional effort.