There are wild claims about the results from doing this. Some people claims this does nothing. Others claim 5 horsepower and 19 foot pounds of torque. As with most things, the truth is probably in between.
However, there is a degree of logic that the modification can help the car run somewhat "better". Newer cars have a lot of electronics on them - lots of sensors to provide information to the car's ECU. If there's electrical noise in these signals, then the ECU *might* make poor decisions while adjusting things like fuel/air ratios, timing, etc. The ECU is the Engine Control Unit or a "computer" that's dedicated to making your car's engine run correctly.
In theory, improving the grounding reduces the resistance of the electrical path, and makes it more likely that the sensors, etc. will return clean information.
So, for $20 and a little wrench time, it's an easy mod to perform even if you don't really notice much difference in performance.
On my TL, I chose to "daisy-chain" the chassis, alternator, engine, throttle body and battery negative post together. You may decide to add or remove some of these locations.
Step 1: Obtain The Cables
These cables come in a variety of lengths. Nineteen inches was the shortest I found, and forty the longest. These cables have plain terminals on both ends. These cables were only available in the ultra-sexy "Henry Ford" black. Doesn't really matter - you'll pretty much never see them - another reason not to spend lots of bucks for 'primo' cables.