Instead of concentrating on the chord sequence of the song, modal jazz is all about scales and modes. This gives more freedom, the accompanying instruments don't have to follow the chords, the soloist can create melodies of his own instead of arpeggiating on and on the same chord sequence.
Inside the scale, the soloist may use intervals, triads, arpeggios, penta and blues scales and patterns derived from the scale.
But the real freedom in modal jazz is to be able to add "out notes", notes that are not within the scale or the mode of the song. It's very important for the soloist to know exactly what notes will make happen his/her intentions and for the accompanying musicians to hear where the soloist leads to or, even to suggest trips outside the basic scale or mode.
Let's take, for example, the A-dorian mode: A, B, C, D, E, F#, G. First, you have to get familiar to the sound of the dorian, then to hear when the improvisation goes outside the dorian and where. Important outside notes that can be used in the dorian mode:
- the minor second - Bb
- the flat fifth - Eb
- the minor sixth - F
- the major seventh - G#
For C-lydian: C, D, E, F#, G, A, B, the same notes of the mode will occur outside the mode but at other steps:
- the flat seventh - Bb
- the minor third - Eb
- the perfect fourth - F
- the minor sixth - G#
It takes some practice to hear all the outside notes. Play as much as you can in the "modal spirit" as a soloist and as an accompanying musician, see if you feel the need to go outside the mode from time to time.