They were all filled of holy spirit, and they began to utter-forth with different tongues according as the spirit gave to them (the words) to precisely-speak-out.
The holy spirit gave to them the words to precisely speak out (the clear utterance of words to speak aloud), and the apostles knew to speak with their own physical tongue and mouth and throat in order to express in the physical realm what the spirit within each one of them wanted them to speak.
They did not understand the words they were saying but they knew that they were speaking supernaturally because the spirit within each one of them was what was giving them the words to speak, not their fleshy, intellectual minds. This was supernatural!
