Titan Shifting, Regeneration
Titan Shifters are humans in possession of the "Titan's power" (巨人の力 Kyojin no Chikara?), which allows them to transform into Titans. They are able to direct the powers and abilities of a Titan with purpose, and unlike regular Titans, retain their human intelligence. It is often noted that Shifters are more powerful than regular Titans.
To transform, they must suffer an injury that draws blood (often self-inflicted), while having a solid goal or belief in mind at the moment of injury. However, being injured by another person is also sufficient. After transformation, their human bodies appear to be in the Titan's nape of the neck, where they control their Titan bodies. Their human bodies are also merged with the Titan's flesh inside, rendering them immobile. It is also visible that their eyes are connected within the body of the Titan, presumably for optic vision outside the body.
If a Shifter's Titan form is pushed to its limit, its human form will emerge from the nape of its neck in a semi-conscious state. This is demonstrated separately by Eren, Ymir, and Annie when their Titan forms suffer defeat. But if a Shifter's Titan form is not exhausted, then its human form can partially emerge from the Titan form in full consciousness
The Titan Shifters also gain the Titan's ability to regenerate, capable of vastly improved healing abilities and even regenerating lost limbs even in their human forms. However, regeneration from severe injuries such as lost limbs can strain their powers and prevent them from transforming for a period of time. It is also shown that they are not safe from regular Titans, and will be attacked even while transformed.
Shifters are resistant, but not immune to the normal absorption that creates mindless Titans. The strength and quality of a Shifter's Titan form degrades with each subsequent transformation, becoming smaller and less controlled each time. As such, it seems that there is a limitation on how longer a Shifter can remain transformed before risking absorption.
Their powers can be transferred to a new host, should they be devoured. This allows an ordinary Titan to become a new Titan Shifter, should they succeed in devouring one