Wheels and tires. Sometimes they run a smaller wheel in the back with a larger sidewall tire. I've seen it done a few times, even considered doing it on my Civic. Never got around to it though.
Edit: Just to clarify, helps reduce understeer. This car is on a staggered setup with different width tires:
F 205/50/15 Kumho Ecsta MX
R 195/50/15 Yokohama S. Drive
First of all, the difference in the pictured car's tires will result in a net
loss of grip. The S.Drive tire underperforms the XS by quite a wide margin. No matter if his understeer is reduced, the car with never be capable of the same ultimate grip as if it had all four XSs because the rear tires suck so bad. The tires also have vastly different properties, from ideal operating temperature to wear characterstics, sidewall flex, resonance frequency, along with various other engineery things that actually do matter. To sum up, his car will handle on a circuit like dog turds compared to the same car with an equal wheel and tire setup front and rear. I honestly have no idea what sort of baseline he set that made him decide he needed to put crappier tires on the rear, but even if he doesn't track it like that, simply taking pictures of it like that should be embarrassing.
Get all four wheels and tires the same size. Set a standard. If you think it needs changed then adjust the
suspension accordingly. Not enough overall grip? Get stickier tires on all four corners, the same size of course. You'll probably have to adjust your suspension to suit these tires, including more roll stiffness on both ends.
Changing your handling balance by staggering your tire sizes should be a last-ditch effort to either reach extreme limits in particular situations, such as maximum front end grip and rotation on a competitive autocross or gymkhana car. Running thicker tires at the front on a proper circuit sounds like a recipe for extremely touchy handling in high speed sweepers because the front has so much more grip than the rear. A Civic with too stiff a rear end will already go around corners sideways, so putting staggered tires on it will make it nearly undriveable.
Choose your tires and set up your suspension accordingly. You have to have the whole package for the best performance - covering up a poorly tuned suspension with ridiculous tire sizes not only shows that you don't know what you're doing, but will also never perform as good as it could.
EDIT: For some reason, getting Extreme Summer tires for drag cars has become popular in the Honda crowd. They get tires not designed for ultimate straight-line acceleration and use them thinking they're a godsend. I honestly have trouble believing this guy is backward enough to use staggered sizes on track and that leads me to assume he is drag racing with it. Poorly, again.