As the name suggests, while hanging off of a bar, the athlete brings his/her feet in direct contact with the bar, or the toes pass over the plane of the bar.  Either placement is correct.  If Toes to Bar are two difficult and scaling is required, Knees to Elbows or Straight Leg Raises can be used as a substitute.  Here’s a video with good explanation and camera angles of the Toes to Bar movement.