Let's oversimplify for the sake of showing the calculation...
There are 4 stocks in the world, A, B, and C, and D, all trading at $25. I invest $100, buying $25 in each.
Now lets say that I know for sure that stock D will go down because I'm going to whack the CEO. So I sell my $25 of D, and put it in A, B and C (A stake of $33.33 in each - let's assume fractional shares are ok.) If Stock D goes to 0, while the others all double. I have a net gain of $100 - $75 from the stocks I already held, but $25 from the new investment, and I've avoided $25 as well.
Let's say you lived in a world where shorting was ok. If instead you invested $25 in A, B and C, and shorted $25 on D, you would gain $75 in A, B and C, and $25 on your short of D. In essence it's the same $100 profit.
Now this only works if A, B and C are also going up.
Let's say that D does move on the event, but A, B and C don't. If you do buy back D after the event, you ultimately are in the same position in both cases. In the shorting case, you have $100 in securities ($75 original, plus $25 of D bought near 0) and in the "sell now buy later" you sell back $25 of A, B and C to buy at the bottom.
Net - you create the effect of shorting.
This falls apart if you want a very concentrated bet, but it's a way that long-only professional money managers who compete against indexes find a way to effectively short.