When controlling the puck around mid-ice or near your own blue-line, the defenseman WITHOUT the puck can help the other D-man by providing a pass option.
While most defensemen will remain open directly in-line with the puck-carrying defenseman across the ice, you can provide additional defensive zone support by dropping back BEHIND the (imaginary) line of your defense partner.
This is known as hinging.
Imagine a door or a gate opening up using the player with the puck as the hinge. Hinging acts as a safety valve for a drop-back pass when the fore-check becomes too heavy.
Hockey Defense Positioning
Great video on hockey defense positioning.