I remember the PPC computers with Open Firmware instead of EFI, there you could easily install os x from 10.0 to 10.4 and back) but anyway now apple using EFI and we have more trouble by downgrading :( what a pity))) Nowadays Apple started updating their EFE by installing Yosemite, and i think that is so stupid, because I started having troubles with my computer even though OS loading faster then on the old version of Loader The problem is in the EFI Bootloader Volume, if you have your mac from 2010 you have an EFI Controller Chip with version for example 1.6 (like BIOS in PCs) already installed in the logic board that supports an old System Drivers, but if you have a New Mac so EFI had been updated to the newer version (Again like in BIOS- bios os 1.6 in 2010 and bios 2.0 in 2014) it would be a terrible headache! if you bought mac in the end of 2013 and your primary OS was OS X 10.9 Mavericks, you can install only this OS(you can update and downgrade between the newer version or Pre installed OS), because otherwise EFI will block an oldest version of OS X such as 10.7x or 10,8x! I had an idea to make my own EFI downgrade with support of 10.8 and so more, but it is a bit difficult to do that because firstly you must have an old OS X and only then the EFI Bootloader. Some days ago I discovered that installing an older version of mac is possible, but no one did it.

Instal the new for mac