[Tom1975]

This question comes up a lot. I know I've gotten it a lot from some of my freinds when I've been explaining odds to them. You're probably going to get a lot of answers which just say 'an unseen card is an unseen card, it doesn't matter if it's in the deck or your opponent's hand', so I'll try to explain a little further.

Let's take the example of you playing 5 card draw by yourself. You deal yourself four clubs, and draw one to attempt to complete your flush. There are nine clubs left in the deck of 47, so the odds are 9/47=19.1%. Agree so far?

Ok, now let's say you play with a friend, you both get dealt five cards, and again you get four clubs and draw one. Now there are only 42 cards left in the deck. The problem now is that we don't know how many clubs your friend has. If he has no clubs, your odds are now 9/42 (21.4 %), which is better than the first example. However, if has five clubs, your odds are only 4/42 (9.5%). He could also have 1,2,3, or 4 clubs. To summarize:

#clubs your friend has - your odds
0 9/42 21.4%
1 8/42 19.0%
2 7/42 16.7%
3 6/42 14.3%
4 5/42 11.9%
5 4/42 9.5%

So if your friend has no clubs your odds are actually better than if you were playing by yourself, but if he has one or more, then they are worse. But, you don't know which is the case, so how can you calculate your odds at this point? You would have to take the chance of your friend being dealt 0,1,2,3,4,or 5 clubs times each of the percentages that you make a flush in that scenario and add them up. So your 'true' odds at this point are
(chance your friends was dealt 0 clubs) * 21.0% +
(chance your friends was dealt 1 clubs) * 19.0% +
(chance your friends was dealt 2 clubs) * 16.7% +
(chance your friends was dealt 3 clubs) * 14.3% +
(chance your friends was dealt 4 clubs) * 11.9% +
(chance your friends was dealt 5 clubs) * 9.5%

If you were sit down and do all the math it works out to exactly 9/47=19.1%, which is the same if you were playing by yourself. Think of it this way - there are five fewer cards in the deck, so if your friend has no clubs, this obviously helps you, compared to you playing by yourself. However if your friend has 1 or more clubs then this hurts you. The effect of these two possibilities effectively cancel each other out, so you can just pretend there are still 47 cards in the deck, nine of which are clubs. I hope this helps, if you still can't grasp the concept, you're just going to have to take it on faith that David Sklanksy and a lot of other smart people have done the math and this is in fact correct.