Shikai in Bleach
In general, Shikai is the first release and manifestation of the true weapon form of a Zanpakutou.
Shikai does enable the user to cast different Kidou-Spells, enforce or add powers of his own and also enable different fighting style depending on the form of the weapon.
Shikai on Legend Of Bleach
The Shikai itself does pump up the user and helps him fight, hence the different aspects are enforced by different types. For instance, "power" aspect increases your attack by 10% which relies on strength.
You need to have at least 20 final tests taken to get the initial event to call out the name of your Zanpakutou. Once you are through, you have to do 4 more final tests to get a high enough power level to meditate (at the training grounds) and achieve the intentional release with intentional use of the power (like Ichigo said, he needed some time after Zangetsu came out to get Getsuga Tenshou).
Types and effects
Power: 10% attack
Kidou: 10% defense
Fire: 10% attack
Water: 5% attack, 5% defense Wind: 10% more damage
Ice: 5% more attack, 5% more damage
Lightning: throws 5% of enemy dmg back at him, 5% more own damage
Each form gives currently one new specialty use.
This means, once you release, you can use 1 new specialty that is otherwise locked.
They are cheap, and will currently deliver direct damage hits that STRONGLY (!!) depend on your attributes.
Fire requires Constitution to wield.
Water requires Wisdom to control.
Wind requires Constitution to endure.
Lightning requires Intelligence to channel.
Power requires Strength to enforce.
Kidō requires Intelligence to manifest.
Ice requires Dexterity to conjure.