Shikai is an energy release technique used by the Soul Reapers in the Bleach manga franchise by Tite Kubo. The essence of Shikai relies on the communication between Shinigami and Zanpakuto. The Shinigami enters their spiritual inner world and call the Zanpakuto's Real Form.
Locked within a Soul Reaper and their Zanpakuto is the power to become even more powerful. When a Soul Reaper unleashes their Zanpakuto 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 Zanpakuto also reveals itself to everyone in its second and more complete form. Some character's Zanpakuto 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 Zanpakuto's owner calls out. The release phrase also reveals the character of the Zanpakuoto. An example of that is Kisuke Urahara using Benihime, whose release phrase is "Awaken." The call differs by the users. This proves that the true power of Benihime is easily locked away and has to be brought out for its full use. Some Zanpakuto remain in Shikai state like Ichigo Kurosaki's.