In Felkin-Ahn model, a nucleophile comes from the least hindered side. The best way to do Felkin-Ahn model is to draw a newmen projection. Then have the nucleophile attack from the smallest group.
Felkin-Ahn model example:
Here, the model shows that the nucleophile prefers to attack from the least hindered side.
In Chelation Control there is always a lewis base or lewis acid is present. Example: Lewis Bases are OR', NR2' or SR' and lewis acids are Li+, MgX+, Zn+2. Since lewis base is present the double bonded oxygen and lewis base form a ring with lewis acid.
Chelation Control example:
Next slide shows that when a Lewis-base or Lewis-Acid is present, Chelation Control gives the major product, by forming a ring in transition state and Felkin-Ahn models fails.
In General: How to choose between Felkin-Ahn and Chelation Control: