The July 2004 Issue of Popular Mechanics did a great job of explaining MDS:
"Here's how MDS works: There are four special 2-piece roller lifters for the cylinders that get disabled. The two pieces are concentric--there's an inner portion surrounded by an outer sleeve. The two parts are locked together by two spring-loaded pins when the engine is in 8-cylinder mode and all the pushrods are operating their valves normally. Under light loads, however, the engine management computer signals each solenoid valve to open and apply high oil pressure to its respective exhaust- and intake-valve lifters. This action forces the two pins in each disabling lifter to unlock the two concentric parts. (To help minimize oil contamination, there’s a magnet at the bottom of each solenoid unit to collect ferris debris.)"
The entire article is very informative and provides some keen insight into the engineering behind the current generation Hemi. Here is a link to the entire article, I hope you find it helpful...
http://www.popularmechanics.com/auto...26.html?page=3