The concept of "run it twice" is that when players go all in, the rest of the board will be dealt two times with the cards left at the deck. The pot is then by splitted into two parts; one for both "runs" where winner of one "run" gets 50 % share or splits ot with the other. This significantly reduces variance in some games.
Imagine that you are playing omaha 4/5 hi and you flop 223. You have 33xx and the opponent has 22AK (you have 1 out).
If you go all in at the flop, you have approximaely 4,88 %. Could it be possible, that the 33xx holding player will get +EV from the run it twice in long run (let's suppose 1 000 000 times 100 € pot)?
Note that if hero hits his 1 outer, the second "run" will be 0 outer, so 0 % 50 € pot.