I've always admired and tried to take something from watching him at work and can't help noticing how his middle and ring fingers slide over the pickup cover as he gently presses down with his index finger. Its less a plucking than it is a press, but never noticed a nail-side attack of any kind. In certain situations I've employed this press technique to get a particular sound, but never thought about it much till I see Jamerson gliding and pushing. Its a great vibe and works well with a punchy, tubey amp sound.