In the Godatta Sutta SN 41.7 Buddha outlines different kinds of awareness release.
These are the immeasurable awareness-release, the nothingness awareness-release, the emptiness awareness-release and the themeless awareness-release.
He subsequently explains which factors their cultivation rests upon - briefly:
1. Immeasurable = An expansive all-pervading awareness of compassion and good will.
2. Nothingness = The monk enters & remains in the dimension of nothingness.
3. Emptiness = The monk considers this: 'This is empty of self or of anything pertaining to self.'
4. Themeless = A monk, not attending to any theme enters & remains in the themeless concentration of awareness.
Each of these are, however, declared secondary to a release called an "unprovokable awareness-release". Buddha does not outline any factors upon which the unprovokable awareness-release rests, so I assume that it's a result of cultivating any of the four aforementioned releases.
Is my assumption correct or is it another (but more advanced) means?
Namastehttp://www.accesstoinsight.org/tipitaka ... .than.html