In his cover of Secret Base - Anohana during the chorus at 1:25 he makes a loud twang-y(?) bass note noise that overall gives the composition a really nice sound. Im wondering what hes doing and couldn't figure it out by myself. It sounds like a pop but it looks like he is thumb slapping. Can anyone help?
First off: I have no idea what the technical/real term for the technique is.
However, if I'm understanding your question correctly, what he's doing is transitioning from a slap on the low E to a Bass and strum thing. But, I don't really hear a "twang". I think that might just be from the hard attack that he does, but it doesn't sound like he's going for "twang" (i.e. snapping the string against the fret board).
Recently I had the opportunity to learn from Mike Dawes about his various percussive techniques that he always relates to a drum set. Essentially, you can have a bass/kick drum, snare, and high hat.
The slap on the 6th, 5th, & 4th strings is considered a high hat. So Mr. Sungha Jung goes from the standard "high hat" slap on string 5/6 to a kick drum with the heel of his palm against the body of the guitar, while simultaneously flicking his middle finger to strum his chord. Also, it looks like while he's hitting the kick drum, he is also doing a snare technique by hitting the side of his thumb just above the sound hole (although, it is difficult to tell from the camera angle).
Basically, you can just flick your middle or ring finger while slapping strings 6, 5, or 4 to allow the note(s) to ring while you still get the percussive hit.
Hopefully that's what you were asking. I'm not 100% on what "twang" you're talking about, but that is my guess on what was meant.