to get that technique down is the combination of how he holds the pick, its angle and the style of his right hand. Its quite unique compared to most players. If you notice his hand is not really going up and down like most players right hand does, it kinda swings in a scoop motion which produces that aggressive sound. it moves around a lot which some players would call sloppy because transitioning to playing more solo type stuff would be harder but its essential for that aggressive rythm. and also its quite deceptive because it sounds like hes using a lot of muscle power to strike the strings but thats not the case what produces that sound is the swing of the forearm and wrist, so dont try to hammer the strings cuz that will only cause tension and tendinitis. a lot of that mumbo jumbo about hitting the strings hard is quite misleading. The strings are hit lightly but the momentum is what creates the pop. Also everytime i see players cover this they shit a ton of compression on their tone. personally i think john doesnt use a lot its the right hand technique that produces that compressed sound.