I know the disadvantages of disabling kext signing, but I was wondering how I can actually disable this on Mac OS X 10.10 "Yosemite"?
To disable the kext signing security setting:
sudo nvram boot-args=kext-dev-mode=1
After changing this setting you need to restart the computer to have OS X recognize it.
It is important to note that the kext-signing setting is global, if you disable it you should be careful to only install system drivers from sources that you trust.
To reenable again use:
sudo nvram -d boot-args
Quoting Apple's documentation:
To enable or disable System Integrity Protection, you must boot to Recovery OS and run the csrutil(1) command from the Terminal.
Boot to Recovery OS by restarting your machine and holding down the Command and R keys at startup. Launch Terminal from the Utilities menu. Enter the following command: $ csrutil enable
After enabling or disabling System Integrity Protection on a machine, a reboot is required.
As of OS X El Capitan, the kext-dev-mode boot-arg is now obsolete.
sudo nvram boot-args="kext-dev-mode=1" # Has No Effect
Source: Kernel Extensions