Elimination vs Substitution
Subtle alterations to molecular geometry can dramatically alter the course of a particular reaction. E2 mechanisms require that the Nfg and Efg be diaxial for elimination to occur.

If for steric reasons a diaxial conformation cannot be achieved there remains the possibility of Sn2 substitution. During elimination the Lewis base reagent is acting as a proton abstracting Brønsted base, in the substitution it is acting as a nucleophile.

For more information look in the Chemogenesis webbook section on competing mechanistic pathways.