I haven't done it but I've been considering it (I'm an M2 and you apply for the program during M3) and have spoken with some upperclassmen who have done it. You take the masters year between 3rd and 4th years, and you also have to have a faculty mentor and be working on research (there is a manuscript requirement) while completing the masters coursework. The coursework is things like biostats, epidemiology, research ethics. The nice thing is it's a tuition free year (Einstein only charges you for 4 years of tuition even if you take a research year/do a masters program). All the people I know who have done the program have done so because they were applying to highly competitive specialties and wanted a research year to build their resume for residency, and this is a more productive route than a normal research year (though it is possible to do that at Einstein as well with a $16k stipend). If you want to do a competitive specialty or match at a big academic center and need lots of research it's a great program.