In the sound head position field you may enter a value for the sound head's new position. The sound head may only move within the time given by the loaded elements, If your last element's Stop Time is at i.e. 19:00:00 (hh:mm:ss) and you type i.e. 20:00:00 then the sound head jumps to the end of the timeline at 19:00:00. It is not intended to load the new elements located at 20:00:00.
During playback you are not allowed to type any position to the "Sound head position" field. Only if OnAir TrackMixer is not in play mode the sound head position can be changed by typing a position.