Here's another example that (I think) explains this stuff (or a very similar concept): "Lord of the Breakthrough" by Israel.
On the second line "(Abmaj7) You are the Lord of the breakthrough, (Bb7) we worship You" the implied key is Ebmaj (or Cmin, its relative minor). If you play those two chords, they'll want to resolve to Cmin (or Eb) - try it!
Instead, the next chord in the progression is C major, the Picardy 3rd. And the cycle goes back again.
Sorry elio, but that's not an example of a true P-3rd cuz that song is in the Key of C. Let me show you:
C / G-C-E
Lord of the breakthrough,C / F-Bb-D
Lord of the breakthrough,Ab / Eb-Ab-C
You are Lord of theBb / F-Bb-D
breakthroug, and weC / G-C-E
worship You.O.k., now let's take out the Ab and Bb chords and talk about them:
Ab / Eb-Ab-C - This chord right here is in the key of Eb, but it doesn't imply Eb. This chord is actually an Fm7 chord w/out it's root. Let's rewrite what it should be so you can see it: F / Eb-Ab-C
Bb / F-Bb-C - This chord right hre is also in the key of Eb, but it doesn't imply Eb. This chord is actually a Gm7 chord w/out it's root. Let's rewrite what it should be so you can see it: G / F-Bb-D
Now, let's put it alltogether with the other chords, and you will see that everyting fits in C:
C / G-C-E
Lord of the breakthrough,C / F-Bb-D
Lord of the breakthrough,Ab / Eb-Ab-C (really: F / Eb-Ab-C)
You are Lord of theBb / F-Bb-D (really: G / F-Bb-D
breakthroug, and weC / G-C-E
worship You.You see now, I don't know the correct theory term for that, but it has something to do with rootless voicings. This technique can be found in the song "Thy Loving Kindness" by John P. Kee. It is at the end of the song where they sing "I will lift up my hands in thy.... Name, ame, ame!" That name part is the same technique, but it acutally has the root being played.
Does that make sense? If it doesn't, I'll have to check my theory book to find the exact term for this, or maybe some other theory scholars out there can help me out.