Well for one thing it's an industrial engine. that is, it was never really intended for a passenger vehicle as such, with the closest useage to that being the stepvan delivery trucks. So Cummins did the ho-hum on dampers. Now I'm no degreed engineer, and not that bright either, but the way I understand it: There are different waveforms that develop along the along longditudinal axis of crank. Pistons transfer combustion 'hits' to crank through rods, and these are occurring in a certain order. From this waveforms develop in crank.
Next is something called sineusoidal wave harmonics, almost similar to playing a guitar chord. You may have noticed something like this at home. You have an electric fan running and it makes a rythmic humming sound. Then the refrigerator kicks on and it too makes a rythmic humming as the motor turns compressor and fan, but at a different frequency. that is, One sounds like hmmmm-hmmmm-hmmmm and the other sounds like hmm-hmm-hmm or whatever, different frequencies. Listen for a few minutes and it's amazing: the two seperate waveform frequencies will harmonize after a few minutes and assume ONE WAVEFORM! Pretty strange effect: 'Sineusoidal wave harmonics', and the sine waves harmonize.
So on the crankshaft a single waveform occurs from each cylinder's combustion, for a total of 4 waves on 4BT. Then the waves begin to harmonize, actually causing a 5th order waveform on top of the other 4. Differing engine speeds can then effect this sort of standing wave and add another, for a 6th waveform. So there's one harmonic at idle, another at running revs, and others that develop at various rpms. I've only heard the brain guys mention up to the 6th order of waveforms. It's more complicated, but I'm not going to go there.
Elastometric dampers can absorb some of the combustion hits on crank and modulate the waveforms somewhat, but they can't do much with the higher orders because of their relative frequency. Fluid Dampers are more able to abosrb the 'higher' frequencies that occur at higher rpms through that disc and fluid mentioned by FarmerO_1. So that's a function of those sineusoidal wave harmonics, as it modulates the highs and lows on the [sine] waveline to cause a harmonic, like a guitar chord, or sympathetic vibrations along the crank centerline.
Any decent engineer worth a hoot can tear what I just wrote to bits for its errors, but it may help to understand what's going on. The 4 cylinder's particular crankshaft harmonics are entirely different to the much more inherently balanced 6 cylinder waves. But at low engine speeds the elastometric [rubber membrane between steel pieces] or the fluid type harmonic dampers can help a lot to take up vibrations on either engine, up to maybe 1,000 rpm's? I don't know, maybe I should have just slept in instead....