Oh, that! Ironically, I was just talking about that sound with the guitarist this evening.
I don’t know what Larry did (wish you had asked this last Thursday as Larry was at our show last Friday and I could have asked him) but I’ll tell you how I do it.
I’m using a subtractive synth (I happened to use the AAS Ultra Analog but any subtractive synth will work) and I have Gig Performer configured so that I can quickly play two adjacent keys on one of my keyboards to generate the same two note low chord with a bit of white noise (the chord has nothing to do with the keys I pressed, it’s being generated by a PlayNotes GPScript function)
However, the trick is that as soon as I start playing notes, a timer goes off and slowly starts increasing the VCF Cutoff frequency. As soon as I stop playing, the script notices that there are no more notes coming in and stops the timer and the cutoff frequency so it’s ready for the next time.
However, I implemented that approach a long time ago. If I was going to do reimplement it I would probably just create a MIDI file that contains the notes and a continuously increasing CC message that would then drive the synth and then use my finger tapping trick to produce the effect.
See for example, this topic