Full Hollowfication is the process to describe when a shinigami gains their inner hollow's full powers. In doing so, the shinigami's strength greatly increases as well as their agility, speed, and stamina. The particular group of shinigami's who have gone through hollowification are the Vizards. There are eight members of the Vizard, whom were all either captain-class or lieutenant-class (excluding Ryun Toshiro). As Aizen once stated, "a normal soul can't contain the prototype" when undergoing hollowification, therefore shinigami have to be the ones who encounter hollowification.
During full hollowfication, a vizard will use their inner hollow's true form and yet still retain consciousness. To obtain this the vizard must seal their inner hollows within a second zanpakuto. Then, after reciting a release command, they can release their full-hollow forms much like an arrancar's resurrección. It is far more draining than using the normal Hollow Mask and for that reason, is used as a last resort. The maximum time limit for any vizard using this state is 15 minutes. They may only use it once a day.
It would seem that every vizard that uses this state uses the release command, "Hollowfy". Also on an interesting note, the zanpakuto used to seal the inner hollow gains a spanish name, that describes the hollow within. The name of a sword with a hollow sealed inside is called a Vacuocorte (spanish, lit., Hollow Blade)