Shikai is energy release technique used by the Soul Reapers in the manga Bleach.
Locked within a Soul Reaper and their Zanpaku- to is the power to become even more powerful. When a Soul Reaper unleashes their Zanpaku- to from it's sealed form to the Shikai release form they invoke a new level of power.
Evidence of Increase
When a Soul Reaper uses their Shikai release the Soul Reaper gains the ability to raise different areas of their abilities. Some may become faster, others stronger, but on top of that the zanpaku- to also reveals it's self to everyone in it's second and more complete form. Some character's Zanpaku- to gets larger in size while others get smaller.
How Does It Work?
The Shikai release is controlled by the wielder and can only be engaged by the release phrase that the zanpaku- to's owner calls out. The release phrase also reveals the character of the Zanpakuo- to. An example of that is Kisuke Urahara using Benihime, whose release phrase is "Awaken." This proves that the true power of Benihime is easily locked away and has to be brought out for it's full use.