IMHO, Blessed Thomas Aquinas makes the explanation easy to understand.
Excerpts from Summa...
Article 5. Whether the Son is in the Father, and conversely?
Objection 1. It would seem that the Son and the Father are not in each other. For the Philosopher (Phys. iv, text. 23) gives eight modes of one thing existing in another, according to none of which is the Son in the Father, or conversely; as is patent to anyone who examines each mode. Therefore the Son and the Father are not in each other.
Objection 2. Further, nothing that has come out from another is within it. But the Son from eternity came out from the Father, according to Micheas v. 2: “His going forth is from the beginning, from the days of eternity.” Therefore the Son is not in the Father.
Objection 3. Further, one of two opposites cannot be in the other. But the Son and the Father are relatively opposed. Therefore one cannot be in the other.
On the contrary, It is said (John 14:10): “I am in the Father, and the Father is in Me.”
I answer that, There are three points of consideration as regards the Father and the Son; the essence, the relation, and the origin; and according to each the Son and the Father are in each other. The Father is in the Son by His essence, forasmuch as the Father is His own essence, and communicates His essence to the Son not by any change on His part. Hence it follows that as the Father's essence is in the Son, the Father Himself is in the Son; likewise, since the Son is His own essence, it follows that He Himself is in the Father in Whom is His essence. This is expressed by Hilary (De Trin. v), “The unchangeable God, so to speak, follows His own nature in begetting an unchangeable subsisting God. So we understand the nature of God to subsist in Him, for He is God in God.” It is also manifest that as regards the relations, each of two relative opposites is in the concept of the other. Regarding origin also, it is clear that the procession of the intelligible word is not outside the intellect, inasmuch as it remains in the utterer of the word. What also is uttered by the word is therein contained. And the same applies to the Holy Ghost.
Reply to Objection 1. What is contained in creatures does not sufficiently represent what exists in God; so according to none of the modes enumerated by the Philosopher, are the Son and the Father in each other. The mode the most nearly approaching to the reality is to be found in that whereby something exists in its originating principle, except that the unity of essence between the principle and that which proceeds therefrom is wanting in things created.
Reply to Objection 2. The Son's going forth from the Father is by mode of the intereior procession whereby the word emerges from the heart and remains therein. Hence this going forth in God is only by the distinction of the relations, not by any kind of essential separation.
Reply to Objection 3. The Father and the Son are relatively opposed, but not essentially; while, as above explained, one relative opposite is in the other.
Article 6. Whether the Son is equal to the Father in power?
Objection 1. It would seem that the Son is not equal to the Father in power. For it is said (John 5:19): “The Son cannot do anything of Himself but what He seeth the Father doing.” But the Father can act of Himself. Therefore the Father's power is greater than the Son's.
Objection 2. Further, greater is the power of him who commands and teaches than of him who obeys and hears. But the Father commands the Son according to John 14:31: “As the Father gave Me commandment so do I.” The Father also teaches the Son: “The Father loveth the Son, and showeth Him all things that Himself doth” (John 5:20). Also, the Son hears: “As I hear, so I judge” (John 5:30). Therefore the Father has greater power than the Son.
Objection 3. Further, it belongs to the Father's omnipotence to be able to beget a Son equal to Himself. For Augustine says (Contra Maxim. iii, 7), “Were He unable to beget one equal to Himself, where would be the omnipotence of God the Father?” But the Son cannot beget a Son, as proved above (41, 6). Therefore the Son cannot do all that belongs to the Father's omnipotence; and hence He is not equal to Him power.
On the contrary, It is said (John 5:19): “Whatsoever things the Father doth, these the Son also doth in like manner.”
I answer that, The Son is necessarily equal to the Father in power. Power of action is a consequence of perfection in nature. In creatures, for instance, we see that the more perfect the nature, the greater power is there for action. Now it was shown above (4) that the very notion of the divine paternity and filiation requires that the Son should be the Father's equal in greatness—that is, in perfection of nature. Hence it follows that the Son is equal to the Father in power; and the same applies to the Holy Ghost in relation to both.
Reply to Objection 1. The words, “the Son cannot of Himself do anything,” do not withdraw from the Son any power possessed by the Father, since it is immediately added, “Whatsoever things the Father doth, the Son doth in like manner”; but their meaning is to show that the Son derives His power from the Father, of Whom He receives His nature. Hence, Hilary says (De Trin. ix), “The unity of the divine nature implies that the Son so acts of Himself [per se], that He does not act by Himself [a se].”
Reply to Objection 2. The Father's “showing” and the Son's “hearing” are to be taken in the sense that the Father communicates knowledge to the Son, as He communicates His essence. The command of the Father can be explained in the same sense, as giving Him from eternity knowledge and will to act, by begetting Him. Or, better still, this may be referred to Christ in His human nature.
Reply to Objection 3. As the same essence is paternity in the Father, and filiation in the Son: so by the same power the Father begets, and the Son is begotten. Hence it is clear that the Son can do whatever the Father can do; yet it does not follow that the Son can beget; for to argue thus would imply transition from substance to relation, for generation signifies a divine relation. So the Son has the same omnipotence as the Father, but with another relation; the Father possessing power as “giving” signified when we say that He is able to beget; while the Son possesses the power of “receiving,” signified by saying that He can be begotten.
IMHO, Blessed Thomas Aquinas makes the explanation easy to understand. Excerpts from Summa...
That's a pretty good read, STF. Thanks for posting. :)