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][1] if you are still confused.
[![enter image description here][2]][2]


  [1]: https://shodhganga.inflibnet.ac.in/bitstream/10603/115802/10/10_chapter%204.pdf
  [2]: https://i.sstatic.net/TcYYF.png