Chiral ligands work by asymmetric induction somewhere along the reaction coordinate. The image to the right illustrates how a chiral ligand may induce an enantioselective reaction. The ligand (in green) has C symmetry with its nitrogen, oxygen or phosphorus atoms hugging a central metal atom (in red). In this particular ligand the right side is sticking out and its left side points away. The substrate in this reduction is acetophenone and the reagent (in blue) a hydride ion. In absence of the metal and the ligand the Re face approach of the hydride ion gives the (S)-enantiomer and the Si face approach the (R)-enantiomer in equal amounts (a racemic mixture like expected). The ligand and metal presence changes all that. The carbonyl group will coordinate with the metal and due to the steric bulk of the phenyl group it will only be able to do so with its Si face exposed to the hydride ion with in the ideal situation exclusive formation of the (R) enantiomer. The re face will simply hit the chiral fence. Note that when the ligand is replaced by its mirror image the other enantiomer will form and that a racemic mixture of ligand will once again yield a racemic product. Also note that if the steric bulk of both carbonyl substituents is very similar the strategy will fail.