re: "... amperage draw goes up as system voltage goes down and visa versa ..."
I think you're thinking of the Power Equation: Power (usually, in watts) = Voltage X Resistance, in which case yes, (available) power is reduced when voltage is reduced. In a simple DC circuit, Ohm's Law applies: E = I X R (E is voltage, and I is current in amps; used for historical reasons). Solving for I gives:
I = E/R
So, a smaller E produces a smaller I (current and resistance are SWAGS for the OD circuit): 1Amp current = 12V/12 Ohms; if voltage drops to 10, then about 0.8 amps is available (for a fixed resistance). The OD solenoid is an inductive load, but once impedance builds it becomes, in effect, a resistance circuit (hence why less current is needed to hold the plunger after retraction, and why a slow blow fuse is used in this circuit). The load on a car is parallel DC, and resistance
decreases as load is added (the sum of the loads, inverted IIRC), so voltage drops and your lights dim because you are reducing resistance, more current is drawn and, since our batteries are 'constant current' supplies less voltage is available (the regulator will compensate to a point, but we know our generators are, uh, 'challenged'). If the solenoid gets less current due to less voltage, it will drop the plunger and deactivate the OD (but if your OD throttle switch is adjusted properly this won't happen until you open the throttles with a blip or downshift).
Voltage is pressure, if your water pressure is reduced you get less water (current) out of your faucet or hose. Here's a cool calculator (enter 2 values, solves for the other two):
http://www.sengpielaudio.com/calculator-ohm.htm