The Charging clip attaches to the back of the NERD where the two brass contacts are located. All you do is press (snap) into place when you want to charge the unit.
When connected to a USB charging port will then charge the internal battery in the NERD. Without that clip, you cannot charge the NERD. The NERD will fully charge from zero in about an hour and the internal NERD battery is then good for about 18 dive hours.