When the 2019 Volkswagen Jetta GLI rolled onto the auto show stage, VW was proud to boast that it offered a load of features from the GTI. Things it hadn't gotten before. Like bigger brakes. The 228 hp engine. And a VAQ differential. But what is a VAQ differential? Why do I want one, and how does it work?

First off, what does it mean? VAQ stands for Vorderachsquersperre. There. That explains that simply enough. What? Sprich kein Deutsch? Well, Google Translate tells me that it means front axle cross lock. Which sounds even more ponderous than limited-slip front differential, but really isn't.

It's an electronic limited-slip diff, but unlike many of those systems, it doesn't just use a brake on one wheel to make the other one spin (and cook your pads). But it's not inside the differential casing like a traditional mechanical LSD or electronic locker. It's like almost nothing else.

Volkswagen said that when it launched, the VAQ system was a first in a front-drive street car. It adds a multi-plate clutch between the differential and the right drive shaft. Basically a Haldex all-wheel-drive center diff. After all, there were probably lots of them on the shelf at VW HQ. But instead of sticking it in the center of the car, it's in the front driveline.

A conventional LSD sends power to the wheel with more traction using gears or clutches inside the case. Everything is inside, hidden, and simple. Want to change just how much power is transferred? Disassemble the whole thing and start changing parts. Like clutch discs, gears, and spacers. Then put it back together. It's impossible to change the way it behaves without a teardown. With the VW system, lockup can be controlled and added or removed as the computer sees fit.

There's a big pack of clutches stuck in-line in the right halfshaft. Right beside the actual differential. Because yes, it still needs one of those to send power to the wheels. It also has a hydraulic pump, because that's what puts the squeeze on the clutch plates. And, of course, there's a controller for the whole thing.

The system can be used for limiting slip. If one wheel starts to spin, the car's computer adjusts the amount of clutch pressure. That makes sure that some of the power is getting to the side with more grip. Adjusting pressure to control how much grip and how much slip there is makes sure that you aren't just sending all power to the grippier wheel. If both wheels can handle some power, both wheels get some power.

There is a limit, though. VW says that the system can handle up to 1,600 Nm of torque transfer. That's about 1,180 lb-ft of torque. You might be thinking that's more than enough capacity for the GTI, but hold on a moment. The engine is rated in torque at the crank. Before it gets multiplied by transmissions and the differential. A six-speed GTI puts that 258 lb-ft through a 3.77:1 first gear and a 3.24 final drive. That's 3,151 lb-ft of torque. So it will still allow for some slip in the first few gears, but it reduces it enough to make sure you can power out of corners.

The VAQ also has the ability to vector torque. Start accelerating out of a curve and the computer uses the clutch pack to send more power to the outside wheel. That helps pull the car. Add torque to the left wheel and the car wants to turn right. Add it to the right and the car turns left. That lets you rotate the car using power and not just lateral grip. More importantly, it reduces understeer and gives the car higher exit speeds. Because off-ramp day, bro.

So that's how the fancy VAQ differential will make your GLI go a little quicker, and one of the reasons why the GLI might just be the most fun to drive front-driver on sale today.