The following animation illustrates the methoxide base-induced elimination of 2-bromobutane. Note the anti-configuration of the reacting groups (H & Br). The product in this case would be trans-2-butene. A similar set of conformations would generate the cis-isomer, but the energy of the cis-transition state is slightly higher due to gauche-like crowding of the methyl groups.
This animation will repeat six times and then stop. To reactivate the cycle press the "ReLoad" button.