http://www.tomshardware.com/news/hdd-sata-power-disable-feature,36146.html
Basically most sata drives do not use 3.3 Volts even though it is available per the SATA specs. The power is sent through multiple pins on most legacy connectors but with the PWDIS feature if the drive receives over 2.1V to 3.6V the drive will be powered down. This is something to watch for with new drives and could even cause a problem if you have a backplane that drives the pin to 3.3 volts.
Basically most sata drives do not use 3.3 Volts even though it is available per the SATA specs. The power is sent through multiple pins on most legacy connectors but with the PWDIS feature if the drive receives over 2.1V to 3.6V the drive will be powered down. This is something to watch for with new drives and could even cause a problem if you have a backplane that drives the pin to 3.3 volts.