Like Richard Pearse just said, I’ve always heard a bent note there. You hear the attack as F, or maybe a few microtones lower than F, and it glides down to E, or maybe a few microtones above E, which is probably why it sounds so wonderfully weird. It’s making a very small and quick dive through the space between the notes.
Problem is you have to fret a note in order to bend it, but that is being played on the open low E string. Bending the neck while playing would be really hard, and anyway it’s bending down instead of up. A whammy bar would be more likely, if the bar is applied while muting the string, then hitting the note while releasing the whammy bar, and doing that repeatedly really fast, something like 180 bpm. That also seems impracticable.
My guess was what’s been suggested above, a very heavy attack that tenses the string so that the attack sounds nearly a half-step higher and then the decay lowers the pitch. You can hear that effect on tabla, like on the concluding note of an energetic raga, when the drummer whacks it really hard. Problem with that is no matter how heavily I apply a sforzando attack to an open guitar string, I cannot duplicate that effect. I’m not sure it could work on a guitar. But if it were to work, it would work best on an open string. Maybe a really heavy-gauge low E would help. Maybe some sort of detuning is involved. <shrug> I can analyze theoretically what needs to be done for the downward bend to happen, but the technique eludes me.