The 0-weight (very light) allows the lifters to collapse at idle and the camshaft appears "smaller". This enables the engines to pass emissions (which the Gen III barely did). Once they see a bit of oil pressure (above idle), they operate like a conventional hydraulic roller. The lifters are similar in design to the high bleed-down Rhodes lifters of years gone by. It also makes the 8.3L sound like a gigantic slant-six.
Thanks for the tutorial about the lifters. That's the only thing that bugs me about the engine is the lifter noise.
That's what gives it character and makes people ask: "Why is your engine ticking/clattering?" Now you can tell them and they can glaze over or just nod in silence with an affirmation of partial understanding.