Sorry. As I was going to say, there are two good articles on this response. Louie Verrecchio writes at Catholic Exchange, and Msgr Charles Pope at the Archdiocese of Washington. They both pick up on this quotation from St John Chrysostom:
If the Holy Spirit were not in our Bishop [referring to Bishop Flavian of Antioch] when he gave the peace to all shortly before ascending to his holy sanctuary, you would not have replied to him all together, And with your spirit. This is why you reply with this expression….reminding yourselves by this reply that he who is here does nothing of his own power, nor are the offered gifts the work of human nature, but is it the grace of the Spirit present and hovering over all things which prepared that mystic sacrifice. (Homily on the Holy Pentecost)The USCCB also has some helpful comments on the response.