Minorisation is a few things. Firstly, it is the belief that any key (major,minor or otherwise) can be 'converted to minor'. Secondly, it is the process of taking this belief and incorporating it into your playing.
The great thing about minorisation is that it can be as simple or as complex as you like, it all depends on how far you're willing to delve into abstract thinking and conversion.
Let us first discuss the simple approach.
If we were to play in the key of G but wanted to 'convert to minor', firstly we could do the most obvious and play in the relative minor (E minor) or, we could use minorisation whereby we analyse the notes in use and adapt to fit them.
If the harmony contains a GM7 chord, we know that the notes are as follows:
Now, quite easily some of you may already be able to notice that there's a minor chord in there.
G [B D F#]
The bracketed section of the GM7 contains a Bm chord and so now we have two options for playing in minor over the G major harmony, the relative minor (E minor), or B minor.
We can take this further still though and not just depend on the harmonic simplicity of a major chord. If we were to take a Bminor7b5 chord for instance:
You can hopefully notice the minor chord sneaking in towards the end of this chord.
B [D F A]
In this case, we have a Dminor chord, or we could even treat the entire Bm7b5 as an inverted Dminor 6 chord:
Taking it further
Once again though, the possibilities do not stop there. I'll take us a bit further away from triads and small extended chords. For those who find chord construction slightly confusing or daunting, this next section may be a bit confusing but I can assure you, it's easier than it seems.
Let us now take an altered dominant as our next chord for 'minorisation'.
Here we will take a G7#5b9 chord:
On inspection, this isn't as easy to notice. The minor chord within is actually nested within different parts of the chord and under enharmonic equivalents. But as always, if we look hard enough we can find a minor chord to convert to.
The minor chord in question is the Ab minor triad.
If we look at the construction of the triad we can see that :-
Ab minor = Ab, Cb , Eb
Granted that in the G7 altered chord we have D# instead of Eb, and Cb instead of B, but essentially they are the same since they are enharmonic equivalents.
That pretty much concludes our look at minorisation. Let me leave you with a few more thoughts and examples.
Minorisation does not have to occur in isolation, it can easily convert an entire sequence of chords to minor;
If we take a standard jazz progression (ii-V-I) in C, we can see the sequence would be:
This can easily be converted:
Dm7 = Dm
G7 = the notes G,B,D,F. D+F being the two notes that convert to Dm
C = The relative major of A minor.
Minorisation all round!