how does that work ? ..... does it shut off spark, fuel and the compression process to the four select cylinders ?

Doc you do realize a quick online search answers a lot of basic questions.
https://www.bing.com/search?q=engin...mp;cvid=04845412522e4df0d9afcea6f9c9352a

My truck has it and you would never know. It runs fine.I wouldn`t be scared of it if that is why your asking.

It actually closes both valves when they drop the 4 cylinders. And yes they shut the fuel and spark off to the cylinders that they keep the valves closed in when they drop them 4 cylinders. Ron

I heard that once it closes the valves the air that is compressed acts like a spring essentially and helps reduce power loss from the dead cylinders. Because the horsepower gets cut way way down when it goes into MDS mode it also adds throttle, but since it is drive by wire you never know that is happening.

I had an 08 Ram with a 5.7 and you could hear the MDS if you listened for it, the exhaust note(mine was all stock) would change a little bit and you could catch the added throttle weird sometimes and feel it. Take in mind I was looking for all these signs, to someone who didn't know or care how it worked you would never know.

Lifters have an electric solenoid attched to them
Said solenoid opens a bleed valve in the lifter. Thus basically collapsing it. Which allows the outer shell to still movebut internally it no longer pushes the the pushrod up to open valves.
Each lifter drops one at a time over a few milliseconds. So you never feel it.

One thing to keep in mind for maint on an mds equipped car. Proper oil changes and viscosity are paramount.

You would get about twice the MPG gain over MDS by shifting into a deeper overdrive gear instead, but a large fraction of non-technically minded drivers associate “weak engine” with shifting.

Now that most engines have Variable Valve Timing
shifting down to a higher MPH/rpm ratio has even more benefit
because the Dynamic Compression can be re-adjusted for its optimum at the lower rpm
unlike earlier engines that had that “off the cam” weak feeling at too low an rpm because “reversion” had started because the fixed intake valve closing past bottom dead center was now too much.