cactiman:
Looking at the SPY/SH chart. They totally mirror each other of course.
How would you trade that??
I wouldn't trade that. That would be pair trading... and while pair trading is an interesting strategy it is something else entirely.
http://en.wikipedia.org/wiki/Pairs_trade
What I am talking about is maintaining a portfolio that is balanced and diversified. Diversified by strategy, diversified by sector, diversified by outlook (bullish/bearish).
e.g. if there are developments that make it likely that some stock XYZ will face considerable challenges going forward then I would consider a bear call spread, a bear put spread (or even a naked long put) but before I would add that trade to my portfolio I would examine my portfolio to see how such a position effects my portfolio's overall delta. If my portfolio is already too bearish (i.e. has too many positions that are bearish) I would hesitate to add the trade. If my portfolio was overall very bullish then the trade would be a good addition to the portfolio... moving it towards neutral.
http://en.wikipedia.org/wiki/Delta_neutral
To tell if your portfolio is too bearish or too bullish you can compute it or you can simply compare the response of your portfolio to market movement.
If the market moved up a few hundred points and my portfolio moved up in a similar manner and percentage I woud say I have too many bullish trades... I am not market neutral.
If my portfolio moves down by a lot when the market moves up I would say I am too bearish I need some bullish balance.
If my portfolio stays the same... then that is just right.
But note: in using such a strategy for portfolio balance... each position needs to have its own justification.
note2: in the above example choosing the bearish trade to exploit AYZ weakness would be balanced by how many similar trades i am carrying. I need to diversify by strategy so that choosing a bear call spread, a bear put spread or a long put... or shorting the stock or SSF would depend on my exposure to similar trades as well as the specific situation for XYZ.
In looking at the SPY/SH graph you do see that if you find yourself with a portfolio that is too bullish a quick and easy balance can be obtained by instituting a bullish position on SH. I am hesitant to do that because the SH position would essentially be a throw-away to achieve delta balance.
To continue ad-nausiam I also want to diversify by sector. e.g. if XYZ is a utility and my portfolio already has a number of trades on utilities it would argue against adding the trade. If I had no utilities it would argue for adding the trade (unless I expected utilities as a group to face problems in the short term future of course)