Syncopation and off-beat chugs
The short version
Syncopation means hitting a spot that isn't the downbeat, and treating the silent downbeat as part of the riff. If you can't count "1 & 2 &" out loud while you play, the off-beat will drift toward the nearest beat as soon as you add gain.
Step by step
- Say the grid out loud. 70 BPM, no guitar: "1 & 2 & 3 & 4 &".
- Chug every 8th, muted. If they're uneven, stop and go back to palm muting. Syncopation over a sloppy grid is just slop with a fancier name.
- Play only the "ands". Leave 1, 2, 3 and 4 silent, palm still on the strings, no pick. Each silence has to last exactly as long as a hit.
- Play only the numbers. Same test, flipped.
- Mix one bar. For example: 1, &, (rest on 2), &, 3, (rest on &), 4, &. Write it down, then loop it with the 1-bar routine.
The usual problem
The off-beat creeps early, rushing toward the next beat, or late, waiting to feel it. A click playing 8ths, not just quarter notes, makes it obvious. Downpicking and alternate picking both work here. Keeping your mute consistent matters more than which direction you pick. See downpicking.