The intuition behind the proof is as follows: If the adversary corrupts the party that first receives the output, then he can provoke his most preferred event E10 by aborting before sending his last message.
because this party is chosen at random, this happens only with probability 1/2; with the remaining 1/2 probability the honest party receives the output first, in which case the best choice for the adver­sary is to allow the protocol to terminate and provoke the event E11.