Everybody had Sum 41 filed under "pop punk", and then Still Waiting kicked the door in wearing steel toecaps. The heavy part right at the beginning is pure rhythm-guitar homework: palm-muted power chords, downstroke-driven, tight and aggressive.
So this clip isolates that intro and points the camera where it actually matters — at the right hand. The chord shapes are the easy half of this riff.
Palm muting, downstrokes and 16th notes: why the Still Waiting riff sounds so heavy
The whole sound is a palm mute. Rest the fleshy edge of your picking hand exactly where the strings leave the bridge saddles: a few millimetres forward and everything turns to cardboard mush, a few millimetres back and the power chords ring out and instantly sound polite. That narrow sweet spot is the difference between "heavy" and "loud". So here is the counter-intuitive bit I tell every student: if the riff doesn't feel heavy, turn the gain down and mute more. Distortion adds sustain, and sustain is exactly what you don't want when you're chugging.
Now the picking, and this is where people get it wrong. Everything that can be a downstroke is a downstroke — that's where the weight comes from. But the fast bits are 16th notes at around 190 bpm, which is roughly 12–13 notes a second, and no forearm on earth downstrokes that. Those are alternate-picked: down-up-down. Downstrokes for the driving stuff, alternate picking for the bursts. The trick is making the upstroke sound like a downstroke — same palm pressure, same tiny motion, barely any pick past your thumb — so the burst stays one machine instead of a strong-weak wobble. Practise at half tempo and keep the palm pressure identical as you speed up. Most people unconsciously lift the mute the faster they go, and the riff turns to mush exactly when it should hit hardest. The Master of Puppets intro is the all-downstrokes version of the same idea, if you want to punish yourself properly.
Tab and Guitar Pro file are in the Patreon post for this video, and if this made your forearm hurt you can throw a coffee my way on Patreon — purely for the riffs. If your neighbours are still waiting for you to stop practising this one: same.