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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.