As stated at our site, "These expressions mean the same thing because everything the Son has is from the Father. The proceeding of the Spirit from the Son is something the Son himself received from the Father. The procession of the Spirit is therefore ultimately rooted in the Father but goes through the Son" (www.catholic.com/library/filioque.asp).
The Trinity is our ultimate model in the self-donation John Paul II speaks of as necessary in personal relationships. Just as the Son receives all that he has from the Father, so the Father gives all that he has to the Son. That means that, although he is rooted in the Father, it can be said that the Holy Spirit proceeds from the Son. The Holy Spirit proceeds from the Son because the Son receives the procession of the Holy Spirit from the Father.
"And the Son" and "through the Son" are slightly different ways of expressing the same reality: The Holy Spirit proceeds from the Father and the Son because the Holy Spirit is rooted in the Father and directed by the Father through the Son.