Negative floats mainly occurs as a result of activities constrains. These constrains prevent the activities to move forward as they impacted from another activities or from the project data date. let’s assume that the project activity is represented by a spring, this spring has a start point and end point (start date and finish date), if this spring was free to move from both directions and you pushed one end of it, it will simply move in the direction you pushed it on. In the other hand, if this spring was fixed from one end (start date or finish date), and you pushed this spring, it will not move, and alternatively, it will be compressed. This compression of the spring represents the negative float on an activity which has a constrain (i.e. must start by or must finish by).