On a SOHC engine, an adjustable cam gear will allow you to move the power curve to a specific area in the rpm band. Like moving your peak power on a high-revving Suzuki to max out at 4000 to 5000 rpm. Typically to improve bottom end power advancing a SOHC engine will do the trick and the converse is true to enhance top end power.
Basically, the job of the camshaft is to open and close the intake and exhaust valves at the "proper time" relative to engine position (piston travel) - to charge the combustion chamber and expel burnt gasses.
However, once you've bolted on performance parts, milled the head or installed a lumpy performance cam, you also alter the optimum cam timing that most sport compacts prefer. What can you do?
By changing the cam timing (retard, advance/overlap), we tune the overall drive-ability and performance aspect of the car. Even stock engines can benefit by dialing-in cam timing/overlap. These are typical variances in production tolerances that when fine-tuned to your particular engine set-up will easily produce added power.
Many factory cams operate on the conservative side of performance due to emissions and drivability standards, meaning there's always more power to be gained by learning how-to adjust the intake and exhaust valve timing.
Just as aftermarket engine management systems manage precise ignition timing, adjustable cam gears allow you to govern the action of the cam, which controls the timing action of the valves to shift peak power points or to tweak timing overlap within the engine cycle. "Overlap" is the point in the cam's cycle where the intake and exhaust valves are open simultaneously. Here's where critical tuning really optimizes performance because you are producing the most efficient charge of air/fuel in and out of the combustion chamber.
In racing, the ability to dial-in ignition and cam timing/overlap for different track conditions can mean the difference between winning and losing a race. Autocross enthusiasts rely on low to mid-range power for tight turns, whereas drag racers need all the power at the top end. This applies to setting up a car for mostly city or highway driving. In either case, adjustable cam gears will optimize the peak power curve so that maximum performance is adjusted to your style of driving.
Count on ZOR quality, don't assume that all cam gears are the same. Variations in material, design, anodizing, and other engineering deviations can affect quality detrementially, watch out for copy cats that are driven by profit, not happy customers.
It's important to note that adjustable cam gears have no direct effect on the duration and lift of the valves, that's the job of the camshaft lobes (profile) as dictated by the cam manufacturer.
If the intake valves open at 10 degrees before top dead center (TDC) and close at 190 degrees after TDC, the total duration is 200 degrees. The opening and closing times can be shifted with adjustable cam gears to rotate the cam ahead a little as it spins. For example, if we advance the intake cam 5 degrees then the inlet valves would open 5 degrees earlier. The intake open timing would shift to 15 degrees BTDC and the intake valve would also close earlier by the same 5 degrees. In the example above, the valve timing would be IO at 15 degrees BTDC and IC at 185 degrees ABDC. With SOHC engines the cam timing events for intake and exhaust occur together so intake and exhaust events move the same amount. Retarding timing works just the opposite.