I’m a bit “late to the party” on this one, because I’m a new member - but I used to play in several thrash bands in the late 80’s.
And we never found any “cheats” for down stroke chugging. In fact in the 80’s because it was such a new sound - commercial amp’s within our budget just were not geared up for some chuggggggggin’ So we had to physically hit the strings harder, rather than super gain - and super deafening noisy feedback whilst playing live.
We used to play Anthrax/Metallica/Slayer stuff.
Anthrax - Madhouse, AIR and GungHo
Metallica - Creeping Death, MOP, One and Harvester of sorrow.
From memory it was all pure excitement/adrenaline/teenage angst (lol) that fueled the speed, with the hardest tune being Creeping Death for some reason! MOP was a real pain - but doable - C.D. just felt a lot harder to play - probably because of the mixed rhythm, chord stabs and trill. We started with Madhouse tho - and that’s a good intro into chuggg playing - because of the sparsity of chuggs in that tune. The machine gun riffs in One - were easy to play - but difficult to time and sync correctly - mainly because our drummer was crraaaap hahaha! The rhythm player in our band was super human - able to play anything downstroke related without breaking a sweat - and he had no tricks to do it - just pure full on muting power!
I think if I had to do all that again it would be easier - with higher quality/cheaper gear and FX - we would be “scuffers” rather than “chuggers!” if that makes sense.