As first discussed in a video by F1 technical blogger Craig Scarborough, then further explained in a video by Sky Sports, perhaps the single biggest development in the Mercedes power unit is that the turbocharger and the compressor sit on opposite sides of the engine; the turbo still sits at the back, but the compressor is at the front of the 1.6-liter V6 and the two are connected by a long shaft that runs through the engine's vee. Locating the compressor away from the turbo allows cooler, denser air to enter the engine, providing more power, and shortens the length of ducting necessary to get the air from the compressor through the intercooler and into the cylinders, meaning less turbo lag. The Sky report also suggests the arrangement lowers the temperature at the rear of the car, meaning the gearbox can be moved closer to the engine to improve balance, and has other benefits such as allowing smaller sidepods and more efficient use of the ERS-H.
Scarborough tweeted a line drawing of the concept, and someone else managed to snap a photo of the slightly obscured compressor on an actual engine. If the images from Ferrari and Renault Sport are to be believed then neither uses the same layout, even though the Magneti Marelli turbo that Ferrari uses did, at one time, place the ERS-H unit between the turbo and the compressor. There is certainly more to Mercedes AMG Petronas dominance than this one trick, but if it's a major contributor then the competition will have to find another way to challenge it: the engines are homologated for the year.
You can check out the two video explanations below, with the relevant matter coming at 5:20 in the Scarborough video.