Wherein they were gathered for when they were assembled, A.V. When they had prayed. When they had finished the preceding prayer. The place was shaken, perhaps by a mighty wind, as in Acts 2:2. The word σαλεύεσθαι is properly used of ships or of the sea agitated and tossed by the wind; so Matthew 11:7, "A reed shaken by the wind."
But it is also applied to the rocking caused by an earthquake (Acts 16:26), which maybe the kind of shaking here meant. In this fresh outpouring or the Spirit, whereby they were enabled to speak the word of God with boldness, they had a direct and immediate answer to their prayer (see Isaiah 65:24).