Per the Nicene Creed:
By the power of the Holy Spirit
Per the Holy Bible:
The angel replied, “The Holy Spirit will come upon you, and the power of the Most High will overshadow you. So the baby to be born will be holy, and he will be called the Son of God."
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This clause of the creed is simply explaining how God the Son took on human form. Well, explaining is probably too strong a word to use. It's more like giving credit without really understanding any behind-the-scenes making of the sausage. For God the Son to take on flesh, it took some kind of special power, and that power was administered through God the Spirit.
Truth is...I don't understand it, but I don't really expect to be able to understand everything about God.