english: It's a demonstration of something in physics that we call the law of conservation of angular momentum, which basically says that if you set something spinning about a fixed axis, that it will keep spinning unless you do something to stop it.
non_english: Ovo je demonstracija nečega što u fizici zovemo zakonom očuvanja ugaonog momenta, koji u osnovi kaže da ako nešto zavrtiš oko fiksirane ose, nastaviće da se vrti ukoliko ne uradiš nešto da ga zaustaviš.