When I pronounce an approximant, a trill, and then a stop, I have an impression that they are discriminated by the strength of articulation (the "strength of articulation" here means the strength of force of the active articulator against the passive articulator, and is distinguished by the diacritics like [k͈] and [k͉] by extIPA). The approximant feels like the weakest, the trill the middle, and the stop the strongest.
I have a voice sample to demonstrate this, here postalveolar, so it's a shift from [ɹ̺] to [r] to [t̚]: Click here
I tried to apply this to other places of articulation, and discovered that the "middle" is a creaky voice. Here is the voice sample to demonstrate this, which is a shift from [a] to [a̰] to [ʔ̚]: Click here
So a creaky voice is a glottal trill?