Engine brakes work by closing the throttle (air flow) and creating a vacuum. Gasoline engines maintain a constant gas flow, and supply power (speed) by modifying air flow (throttle). The moving parts have to work much harder when there is a resistance, thus forcing the atv to slow down. On a normal run of the mill gas engine, you let off of the throttle and the air flow goes to the idle setting, let off of your throttle and it closes it completely until the atv is stopped or slowed significantly, and then opens the throttle back up to idle setting. Its a pretty common feature in nearly every single engine (other than 2 stroked) these days. It not only helps you control the atv when you need to quickly stop, but it saves quite a bit of fuel (which is why they are in nearly every vehicle as well).

Hope this helps (I noticed there were two posts about it so I thought I would chime in).

Your ATV has an automatic clutch connected to a CVT of the variable-diameter pully type. You can see it in action here.

From the video it looks like a clutch activated when a certain engine speed is reached, combined with the mechanism that adjusts the pulleys. It is probably activated by centrifugal force, like the clutch on a chainsaw.

What they call EBS could be a simple feature of a mechanical centrifugal clutch with two trailing shoes. If the wheels start driving the engine instead of the other way around, the trailing shoes become leading shoes with the friction preventing them from dis-engaging.

After reading Roland's post, I decided to look into your specific ATV (Having been on thousands of mountain rides, I have never actually driven a Polaris, just not the brand for me). Turns out Polaris actually got creative with their use of the term for engine braking and it is much more akin to ADC (Active Decent Control), which is what most other manufacturers use for the same term that Polaris is using. Even the description (With our unique Engine Braking System (EBS), the engine assists with braking on hills and steep inclines) they use is not correct for an EBS, as an EBS is not specifically designed for descent. I also think the word unique gives it away that it is a marketing ploy in this case. Isn't it annoying when a company designs a "unique" system that requires a special alignment tool, a disposable part, and under most situations a trip to the shop for maintenance? Anything for a dollar these days! You can see the parts that make up the system here http://www.purepolaris.com/en-us/Rzr/pages/detail.aspx?ItemID=2200953(PolarisPGACatalog)

Unfortunately, that belt is considered a service part, which equates to being a disposable.

For the correct usage of EBS (as according to any gasoline engine mechanic), see this link http://en.wikipedia.org/wiki/Engine_braking

Note that the term is used interchangeably for gas and diesel engines. However, petrol engines uses vacuum, on diesel engines it refers to the noisy "jake brake" which actually releases compression.

I know this is a post about the ATV brakes but I received the nicest surprise in the mail today (even though I have been watching for them!).....you guessed it....the onions arrived today!!!! Yeay!!!! Thanks, Anna and tell your dad I said thanks, too. Yeay, again!!!!

For a detailed description of how your transmission works, see this PDF document. This is for the RZR model, but from the video I posted in my previous comment the sportsman looks very similar.

It seems my guess was good. It is indeed a relatively simple mechanical system, governed by centrifugal force. Which is a good thing since simplicity means there is less that can go wrong!

Almost every scooter or moped these days uses the same system (if somewhat smaller) The big advantage of a CVT is that it allows the engine to stay in its power band (the rev range where it works optimally) over a large range of vehicle speed.

@Robert:

You wrote:

Isn't it annoying when a company designs a "unique" system that requires a special alignment tool, a disposable part, and under most situations a trip to the shop for maintenance?

Unlike a gearbox where the forces are transferred by the shape of the gears (and which needs a separate clutch), a belt-drive CVT like this transfers force by friction. Since friction depends on contact pressure, the belt needs to be under a significant amount of tension to prevent slipping. And friction leads to wear. Plus belts are usually made of rubber with steel cords, aramid fiber or even carbon fibers inside for strength, and rubber wears faster than steel. Apart from the continuous variation in gear ratios that this drive makes available, it also reduces vibration and noise. So it is hard to make something like this without the rubber belt. And it's not really unique. You'll find it in millions of scooters and mopeds the world over.

These V-belts are pretty common parts. They're used a lot in all kinds of machinery. Replacing them is usually not that difficult; you have good access once the cover is off. Checking the seal on the cover is important. Some mud would make a really nice abrasive to wear down your belt extra quickly.

There is an alternative. The steel pushbelt CVT, developed originally by Van Doorne in the Netherlands. This uses a belt made out of steel links and held together by steel bands. Interestingly with this steel belt is it the push side that does all the work, because there the steel links are pushed together into a column. It lasts much longer and was developed for use in cars (where it needs to to transmit a lot more power than in an ATV). It is therefore also heavier and more expensive; the >150 belt links are hardened steel fabricated to narrow tolerances. It also need lubrication. You can see a demo here. An ATV engine doesn't generate enough power to justify using this steel belt technology.