[QUESTION] Trying to figure out a certain technique

I am trying to learn rolling in the deep arranged by sungha jung but there is a certain technique I can't seem to figure out , it starts from 0.32 in the video where he slaps the frets, I assumed it to be some slap and flick technique but he isn't hitting any lower strings and instead plays the 6th string and slaps the 4th and 5th, I can't seem to figure out how to do it no matter how hard I try and can't find any videos online, do you guys have any idea how to do this?Heres the video: https://www.youtube.com/watch?v=wpsj8-O0vOY , and the tabs: https://ift.tt/2Kcr4fw for some context