John Piper explains with a neat illustration:
Now let me suggest an analogy to illustrate the experience of the Spirit before and after Pentecost. Picture a huge dam for hydroelectric power under construction, like the Aswan High Dam on the Nile, 375 feet high and 11,000 feet across. Egypt's President Nasser announced the plan for construction in 1953. The dam was completed in 1970 and in 1971 there was a grand dedication ceremony and the 12 turbines with their ten billion kilowatt-hour capacity were unleashed with enough power to light every city in Egypt. During the long period of construction the Nile River wasn't completely stopped. Even as the reservoir was filling, part of the river was allowed to flow past. The country folk downstream depended on it. They drank it, they washed in it, it watered their crops and turned their mill-wheels. They sailed on it in the moonlight and wrote songs about it. It was their life. But on the day when the reservoir poured through the turbines a power was unleashed that spread far beyond the few folk down river and brought possibilities they had only dreamed of.
Well, Pentecost is like the dedicatory opening of the Aswan High Dam. Before Pentecost the river of God's Spirit blessed the people of Israel and was their very life. But after Pentecost the power of the Spirit spread out to light the whole world. None of the benefits enjoyed in the pre-Pentecostal days were taken away. But ten billion kilowatts were added to enable the church to take the light of the gospel of the glory of Christ to every tongue and tribe and nation.