The silicon α and β effects arise because 3rd period heteroatoms can stabilize adjacent carbanions charges via (negative) hyperconjugation. 
In the α effect, reactions that develop negative charge adjacent to the silicon, such as metalations, exhibit accelerated rates.  The C&ndash;M σ orbital partially overlaps the C&ndash;Si σ* anti-bonding orbital, which stabilizes the C&ndash;M bond.  More generally, (i.e. even for "naked" carbanions) the Si σ* orbitals help stabilize the electrons on the α carbon.
In the β effect, reactions that develop positive charge on carbon atoms β to the silicon accelerate.  The C&ndash;Si σ orbital partially overlaps the with the C&ndash;X (leaving group) σ* orbital (2b):   This electron-density donation into the anti-bonding orbital weakens the C&ndash;X bond, decreasing the barrier to the cleavage indicated 3, and favoring formation of the carbenium 4.