What is meant by the Imam's departure in the Hadith: "Whoever prays with the Imam until he departs, it will be recorded for him as spending the night in prayer"?

By this act, you will, Allah willing, attain the reward of Qiyam (night prayer). This is achieved simply by the Imam completing his prayer. So, if a man prays with the Imam and continues in his prayer until the Imam departs from his prayer, he should not leave before him. It is also possible that he remains in his prayer place until the Imam departs from his prayer place.