I always thought it was a wammy bar dive or the tuner being rolled down quickly. But I watched that video and saw it was a pedal. I am sure live you could pull it off a few different was and come out close.
There was a video of it on YouTube from
MSG I think and he was stepping on a boss pedal, don't know which one though but I think it was blue. I've been using a line 6 pod bender set to drop 14 steps. The riff is in E but I think the low note is a D.
I read an interview about the gear Dan used in the studio for this record and he said he used a strat with the p/u selector on the first position for the main riff. He said he just depressed the whammy bar to achieve that effect.
LOL - I remember my son playing that in his school band a few years ago when he was about 12. He used his Gretsch with a Bigsby on it, and did the dive with his LEFT hand reaching over and depressing the Bigsby arm while he plucked the string...
EDIT: Found a video of them practising - My son is the guitarist closest to the camera... They ended up playing this song for former Australian education minister Peter Garret (ex Midnight Oil singer)...
Boss PS-5 super shifter, but there's more than one way to come close. And also you'd be surprised how many different guitars/amps are used on a record or single song for that matter that the guitarist normally isn't seen playing live with.
Even the solo to Stairway to Heaven was played on a telecaster, yet how often is Page seen playing one for that song live.