A Mask used to access ones Hollow Powers
A Hollow Mask is a special item gained from the process of Hollowfication. The Mask Contains the Hollow powers which are obtained in the hollowfication, to activate the powers one must don the mask. The mask can only be worn for a certain time limit which varies from person to person before it will crumble and leave the user Exhausted. Through training the time on can don the mask can increase, Ichigo started off with only having four seconds, after some training he managed to use it for eleven seconds, whereas people like Mashiro could use it for hours, since she initially obtained her powers.
When the mask is donned the user's eyes turn hollow like, their voices become muffled and distorted. Their Strength and speed are increased by an insane amount and they gain unique abilities such as Cero.
When the mask crumbles and disappears from the user it will take some time for them to bring the mask back again. With the mask is on the wearer is invulnerable to pain, similar to a Hollow; and can keep on fighting no matter what damage is down to the body.
Sosuke Aizen was the first to test out the process and used Shinji and his group as a test subject and said it as a fail experiment.