So I haven't been able to find any resources on how to do this, and I'm wondering how does Sungha make the click noise
when he plays? Like he strikes the strings and they click, and sometimes he also plays a note. Example is like in let It Go, after the smooth introduction.
Here's an alright video:

The technique really isn't that difficult, and once you get the basics it becomes second nature. essentially you just hit your thumb against the bottom string (or A string, if you want to play that string immediately after), which causes the string to hit agaisnt the end of the fretboard, causing a "click". A couple things to know is that your hand can't be completely facing down (classical technique), as you need to be able to swing your thumb down, and in order to do that comfortably your hand should be facing the fretboard slightly (whatever's comfortable). Another thing is playing chords/ notes at the same time as the click. As plucking and hitting your thumb down at the same time is extremely difficult, you instead have to flick your fingers out as you strike the bottom string.

This technique is extremely common in modern/contemporary acoustic guitar, guys like Andy Mckee, Mike Dawes, don ross etc are all great examples of players who use this. I personally learnt this technique through playing songs (mainly Andy Mckee ones). Any of Sungha's pop covers would make great practice.
I think Michael Chapdelaine does this to. I believe Sungha mentions him as well.
This thread is somewhat old, but if you still want an answer, I've tried to do this as well, only I first found it in John Mayer's music. If you want to do the "click" and play notes simultaneously, hit the bottom E string (I guess it doesn't really matter which string, as long as it's lower than the notes you want to play in the rhythm/melody), while you hit the other strings. Mayer makes kind of a claw- fretting thumb facing upward and hits the string on the downbeat whilst striking the index and/or middle finger on the higher strings. I've found that it's now a constant habit I do and use it to add some percussion when I play alone or help myself keep tempo.

Try watching this. https://www.youtube.com/watch?v=JlSVfe4cHsA
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