Everyone who’s ever driven an automatic has probably figured out that the ‘D’ on the shifter stands for ‘drive.’ How an automatic transmission works is a bit complicated. It uses a torque converter to transfer power from the engine to the transmission input shaft via a planetary gearset, employing a complex combination of mechanical, fluid, and electrical engineering to determine which gear the car needs to be in and shift without requiring any input from the driver. It’s an ingenious design that has been endlessly refined and optimized since its inception.

That said, there are still many drivers who prefer the tactile feel of operating a manual. The debate as to which of these transmission styles is better is an old and well-trodden one, but there are definite benefits to both. Manual vehicles require the driver to press the clutch and shift gears by hand. Most of these stick shifts look pretty similar, with the different forward gears being represented by numbers and an ‘R’ for reverse–but that isn’t always the case. There are some shifters out there that also have a ‘D’ printed on the handle as one of the available gears. This isn’t the same as the ‘drive’ setting on an automatic, but it is actually an ‘overdrive’ setting that is designed to be activated when you’ve already pushed the vehicle as far as it can go in its top gear. Drivers who have a car or truck with this designation might be curious a little more about what it is and how it works.

Overdrive might sound like some ultra-powerful driving mode that injects rocket-fuel into your engine and gives you a tremendous boost in power, but unfortunately, the truth is a lot more mundane. In fact, because of the gearing, the setting actually reduces the amount of torque delivered to the wheels, resulting in slower acceleration.

In practice, Overdrive is a setting that essentially functions as the highest gear in your vehicle. So, if you have four numbered gears, Overdrive would act as the fifth, and if you have five numbered gears, it would act as a sixth–though it’s actually a little bit different from a normal high-gear in terms of the actual mechanics. The top gear in your car or truck will normally operate with your transmission and driveline turning at the same speed in a perfect 1:1 ratio with the engine’s RPM. Overdrive lowers the gear ratio so the transmission and driveline can spin at a higher speed than the engine’s RPM. It’s designed for use on clear highways and freeways with low traffic, allowing you to travel at high, consistent speeds.

Because the transmission and driveline are spinning faster than the engine, the setting actually lowers the RPM your engine produces at higher speeds, which has a few useful effects. Overdrive lowers fuel consumption, giving you more miles per gallon, allowing for quieter and smoother operation at high speeds, and causing less wear on the engine itself. This makes it a great setting for commuters putting a lot of highway miles on their vehicle, as it can save them at the mechanic and the pump in equal measure. In short: Less power, more efficiency.

While that “D” definitely stands out as an oddity on a vehicle with a manual transmission, many automatic cars and trucks have Overdrive functionality too, and it essentially functions the same way, just with a little less driver input.

Sometimes you might have two ‘D’s on your automatic shifter, a regular one for the standard Drive setting and one with a circle or “O” around it to signify that it’s an Overdrive setting. Other vehicles may give you a button to activate the Overdrive setting. This is sometimes located on the console or dashboard, and sometimes on the shifter handle itself. You simply press it in order to have the mode automatically activate when you’ve reached a sufficient sustained speed, just like how the transmission automatically shifts through all of the other gears. Some of the vehicles with this button-based activation method may also have a dashboard light that says “OD On” or “OD Off” to indicate if the mode is currently active.

Whether your vehicle is manual or automatic, it’s important to be able to turn the mode off in certain driving situations. If you need to tow a heavy load, such as a trailer or camper, then you might need that extra horsepower–especially when accelerating to pass other vehicles. It may also sometimes be beneficial to turn it off when driving on roads with steep grades that require a certain amount of oomph from the engine to maintain a consistent speed.