interesting guitar effect seems to slow rate of attack. how does it work?

[Michael Shiloh]

One of my students built this kit which had a very nice effect, that of
slowing the attack of a note:

http://www.buildyourownclone.com/swellinstructions.pdf

It sounds like a magnetic tape recording played backwards.

I can't figure out how it works and wondered if any of you might.
Perhaps a slowed attack envelope through IC1 controlling T1 which shunts
part of the signal to ground?