Those are the EHW (extended hit window) fret trick glitch ticks mentioned in the description discovered by Jilde near the beginning of this year (on this song coincidentally). What you're essentially doing is tricking the game into completing the sustain early on a different fret by abusing the rather jank EHW mechanics. The execution for it is to strum on the sustain (within the following note's hit window) and input that note while still holding the sustain's fret, which is obviously impossible under normal circumstances. The earlier and more precise you perform this, the more glitch ticks you earn obviously. Max EHW fret trick glitch ticks require both a frame perfect strum like regular glitch ticks, but also a frame perfect fret of the following note on the next frame which makes them significantly harder to optimize.