I have no idea how they are getting that effect but I really like the format op& the question. One of these every couple days might be fun and educational.
Howz about a leslie ramped up fast? If I had one I'd give it a try it for this sound. In real life I have the wah, vibrato and even Flanger. Maybe I'll give it a try for myself.
in the mid 70 there was a pedal called a Univox univibe that I had that had a 5 position rotory knob that had a wah effect similar to an auto wah with a rate control and you used the pedal to create a sweep effect over the auto wah it sounded very similar to this effect
I'm curious about what sounds you get.
I thought about a Mutron III in the down position, with in front of it an tremolo pedal.
Now... i've got an original Mutron but... hehe not a tremolo
I could pick a chord and quickly turn my volume pot.
That must do the trick and if it sounds cool i must buy a tremolo.. or maybe i already should!
Original Mutron! I'm Jealous I'll trade you a tremolo pedal for it
I think there's a similar effect in a couple of Bread songs, such as at the beginning of "If":
And in "Diary" in the bridge (about :35 in):
I always thought it was an envelope filter or wah perhaps with tremolo (or leslie) played at the same time.