Think about how the PPh3 first reacts with the epoxide - it does a nucleophilic attack on one of the carbons from below the epoxide ring to give the intermediate shown below left (I have drawn the P substituents as Me for clarity) This then rotates to form the betaine (4-membered ring with P-O bond shown right) putting the Me groups cis to each other. The betaine then eliminates triphenylphosphine oxide in a [2+2] process with the Me groups locked in the cis configuration hence the product alkene. Better diagrams are here if you are still confused.

Think about how the PPh3 first reacts with the epoxide - it does a nucleophilic attack on one of the carbons from below the epoxide ring to give the intermediate shown below left (I have drawn the P substituents as Me for clarity). This then rotates to form the oxaphosphetane (4-membered ring with P-O bond shown right) putting the Me groups cis to each other. The oxaphosphetane then eliminates triphenylphosphine oxide in a [2+2] process with the Me groups locked in the cis configuration hence the product alkene.

Better diagrams are here if you are still confused.

Think about how the PPh3 first reacts with the epoxide - it does a nucleophilic attack on one of the carbons from below the epoxide ring to give the intermediate shown below (I have drawn the P substituents as Me for clarity) This then rotates to form the betaine - 4-membered ring with P-O bond - putting the Me groups cis to each other. The betaine then eliminates triphenylphosphine oxide in a [2+2] process with the Me groups locked in the cis configuration hence the product alkene.

Better diagrams are here if you are still confused.

Think about how the PPh3 first reacts with the epoxide - it does a nucleophilic attack on one of the carbons from below the epoxide ring to give the intermediate shown below left (I have drawn the P substituents as Me for clarity) This then rotates to form the betaine (4-membered ring with P-O bond shown right) putting the Me groups cis to each other. The betaine then eliminates triphenylphosphine oxide in a [2+2] process with the Me groups locked in the cis configuration hence the product alkene. Better diagrams are here if you are still confused.