Hollowfication is a forbidden processes where a Soul Reaper obtains powers similar to a Hollow.
Hollowfication is a process which causes a Shinigami to gain hollow powers, It is still currently unknown what causes the process to begin but it is known that emotions such as Anger cause the process to go faster. Staying calm can slow the process down.
The Vizards were the first Shinigami to go through Hollowfication, This included Shinji Hirako, Love Aikawa, Kensei Muguruma, Mashiro Kuna, Rojuro Otoribashi, Hiyori Sarugaki, Hachigen Ushoda, and Lisa Yadomaru. The process was caused by Aizen.
Later Kurosaki Ichigo goes through the process of Hollowfication as well, he also learns how to master his inner hollow with the Vizard's Help.
Once a person goes through hollowfication they gain immence power and speed, they also gain a lot of Hollow abilities such as Cero. Most of the other powers concerning people who have gone through Hollowfication have still not been revealed. It is also known that a person has much more pain resistance, an example is when the Shinigami fought Hollow Kensei their attacks hardly affected him.
In order to acess their powers, one must don their hollow mask, this will allow them to use their hollow powers for a certain amount of time, the duration varies from person to person. In order to control their hollow, one must defeat their Inner hollow.