The VOES works in conjuction with the ignition module to control ignition timing. The ignition module has two timing curves - one is for idling or cruising and the other is for WOT and/or for powering up for high loads (like steep hills). The VOES switches the ignition module between these two pre-programmed advance curves based on manifold vacuum.
The VOES has two connections and one adjusting screw. The VOES is connected to the carb (manifold side) through a vacuum hose and monitors the manifold vacuum level. It is also connected to the ignition module. It has two black wires from the internal switch. From the inline connector, there is a (purple or purple/white) single wire sent to the ignition module and a second wire sent to ground. The VOES switching point is set by a concealed screw. This causes the ignition module to switch between the two advance curves based on a set point level of manifold vacuum.
The adjusting screw is inside a sealed opening on the VOES. You must dig out the silicone sealing compound in order to make any adjustments. Remember - When testing your adjustments, you must also seal that opening with your thumb (or other air-tight sealing compound or tape) in order to prevent air leakage through the screw adjuster.
The only function of the VOES is to switch between the two advance curves that are programed into the ignition module. Different ignition modules have different curves. Those curves have anywhere from 5 to 18 degrees difference in the advance settings between them. But that's another discussion.
The manifold vacuum directly controls the VOES, but the throttle position (and/or it's aggressive changes) indirectly alters the VOES (thru changes in the manifold vacuum) by altering the carburetor throttle plate.
When the engine is running at idle, when lightly accelerating or when using a steady cruise throttle, manifold vacuum is high (the throttle plate is mostly closed) and the VOES switch is ON, causing the ignition module to use the MORE ADVANCED CURVE.
When the engine is not running or when the throttle is quickly opened to accelerate or to satisfy a heavy load (steep hill, extra weight, etc.), the manifold vacuum drops and the VOES switch goes OFF, causing the ignition module to use the RETARDED (OR LESS ADVANCED) CURVE. If you let off the throttle (when you reach your speed or top the hill), the manifold vacuum rises again. The VOES senses this and will again switch ON, causing the ignition module to return to the MORE ADVANCED CURVE as the load on the engine becomes lighter.
Running without a VOES causes the ignition to run on ONLY the less-advanced power curve. The ignition module cannot switch curves. Unless you have a highly modified engine, this will hamper smooth engine operation on light acceleration and reduce fuel savings during cruising.
Also note, if the VOES switching point is set too low, it will remain on the more advanced curve too long under mid-load acceleration and pinging will occur. This is the reason that engines with upgraded performance should have their VOES switch point increased (switch at a higher vacuum level) to better match the capabilities of the engine.