The Start method is called when the scene loads (more details of call the procedure find here https://docs.unity3d.com/ru/530/Manual/ExecutionOrder.html
). It is filled with amendments to the initial rotation of the character' joints.
In Update, the first condition verifies the presence of the recognized user. Nuitrack is able to recognize up to 10 users at a time, so CurrentUserTracker.CurrentUser returns 0 if there is no one in the view of the sensor to recognize, otherwise the user number of the first logged in view according to the queue rule (that is, when leaving the scope, the current user will be the next one who entered, regardless of when he entered).
CurrentUserTracker.CurrentSkeleton - receives the skeleton data of the current user.