Backing up your partner (on overheads)

When your partner jumps up to retrieve a lob and execute an overhead, you should move forward, ahead of him, right after he hits his shot to cover the possible return from the opposing team.

Since your partner is recovering (landing, and with his racquet still in the follow-through phase), you should be prepared to take the next ball just in case it is returned right away by the opponents. 

You do that by moving up towards the net to cut the angles.