Turn your car into a ROCKET with this amazing device that reprograms your ECU for maximum torque while reducing fuel consumption.  But only if you have a vivid imagination.
I actually love quack stuff like this.  It's probably come from the same factories that made the clone ELM 327 diagnostic units, but have now turned their attention to the much more profitable bogus OBD tuners.  The component cost is very low and the construction can be done on one thin single sided PCB.
Their operation is as follows.  The 12V supply at the OBD2 port is used to derive a 5V supply to power a low cost microcontroller.  The unit fakes communication with the cars ECU by flashing and then flickering the LEDs like an OBD2 diagnostic unit.  In reality no communication is taking place because the unit is not capable of changing the state of the car's low impedance networks.  Instead it just senses activity on them (if it even does that) to control LED activity.  It's possible that the only reason the network pins are involved at all is to make the unit look plausible to an untrained eye.  Although some cars can have some parameters changed via the OBD2 (On Board Diagnostics) port for easy dealer tweaks and the more extreme geeky re-tune garages, the concept of a single tiny device that can determine the brand, model, engine type and any subsequent manufacturer updates, then look up a chart with settings and limits and then somehow adjust the settings for efficiency based on the drivers style is somewhat debatable.  It would be a very clever piece of software and the chance of it being found in a $3 dongle is low.  There would also be the huge risk of literally bricking your car, and if it was being driven at the time that could be dramatic.

That's not to say that these units can't affect the performance of your car though.  All it takes is a solder bridge or wrong component to totally take down a network and result in very odd behaviour.  They also have a bit of a random current draw, possibly approaching nearly 100mA even when your engine is turned off, so some of them are pretty much the equivalent of leaving an interior light on 24/7.

That said, they achieve their intended purpose with aplomb, and have just enough tech to pass themselves off as plausible.  Very much a worthy addition to your collection of dubious devices, especially if you pay less than $3 all in.  Also a rather neat case and plug assembly that is begging for your own OBD project, whether that's a bus monitor, security bypass device (uh-oh) or simply a convenient source of a continuous 12V supply.

Here's a generic eBay search link.  Target price is $3 or less:-
https://www.ebay.com/sch/i.html?_from=R40&_nkw=OBD2+petrol&_sacat=0&LH_TitleDesc=0&LH_BIN=1&_sop=15

Here are a couple of links to the YouTube garage department to inspire you to get more intimate with your ECU.

South Main Auto.  For Eric O's start to finish fault deduction.
https://www.youtube.com/channel/UCtAGzm9e_liY7ko1PBhzTHA

New Level Auto.  For Keith's automotive fault investigations.
https://www.youtube.com/user/ONBOARDTECH333KMD

Pine Hollow Auto Diagnostics.
https://www.youtube.com/user/motoYam82

If you enjoy these videos you can help support the channel with a dollar for coffee, cookies and random gadgets for disassembly at:-
http://www.bigclive.com/coffee.htm
This also keeps the channel independent of YouTube's advertising algorithms allowing it to be a bit more dangerous and naughty.