Another option expiration week is upon us, and we are knee-deep in the related ritual of gauging, gaming and maneuvering of positions that precedes every third Friday of the month. I've never been one to try gaming the bias or direction of expiration day for speculative purposes, but I do keep aware of what levels still retain high open interest.

Understanding the potential impact option expiration can have on trading activity can help you interpret the action and not be overly influenced by what are likely to be temporary price dislocations or otherwise unexplainable surges in volume.

Indeed, the one reliable byproduct of option expiration is an increase in trading volume. Higher volume is a result of investors needing to close out expiring positions. The challenge then is to determine if, how and why this increase in volume can influence price action in the underlying shares and in the overall market.

Currently, the likelihood that this week's expiration will create a directional bias across the broad market seems remote. This is because the indices are now trading in the middle of their recent trading ranges, and the remaining open interest in the nearby strikes is relatively low and fairly balanced. As of Wednesday morning, with the S&P 500 Index trading at 1233, the 1230 and 1235 strikes each have about 25,000 contracts still open in both the puts and calls. The 1225 strike has the peak open interest, with 129,000 calls and 106,000 puts still open. This level could act as a magnet, but with the configuration being in balance, I don't foresee any buy or sell programs kicking in.

Remember, the SPX options actually cease trading on Thursday's close and are settled on the basis of Friday's opening price. In the Nasdaq 100 Trust ( QQQQ), the strikes of peak open interest are the $40 call with 232,000 contracts and the $39 put with 168,000 contracts. This could keep the ETF, currently trading at $39.60, bound between those strikes.

Roiling and Rolling

Watch for large institutions that had established sizable long protective put positions. As I noted in this recent article on option volume , investors have been buying put protection even as stocks rallied over the last two weeks, seemingly in response to the market's failure to take out the July highs by reducing risk.

Some possible adjustments we could see this time include rolling into the October contracts or simply unwinding existing positions by selling out long holdings and their related hedges. The hedges most likely take two forms: covered calls, in which call options are sold short against the underlying security, or married puts, in which puts are purchased as insurance against the long holdings. In either case, the unwinding of positions would create moderate downward bias and tend to put pressure on option premiums, as the desire to reduce exposure drives prices toward a short-term price equilibrium.

The Pinning Dynamic

The dynamics that can make stocks gravitate toward strike prices, creating support and resistance levels, or the phenomenon known as "pinning," work as follows. Imagine I bought 500 shares of XYZ stock at $16 and I'm short five $17.50 calls. The stock is currently trading at $16.50. For now, I don't need to do anything except hope the XYZ shares rally to $17.50.

Assuming the stock does rise, I could close the position by selling the stock (which creates downward pressure) and buying to cover my short calls. The calls I buy are sold by someone else, who now in turn is short the XYZ calls. They might in turn buy shares of XYZ to offset their short calls (creating upward pressure). The result is to drive the underlying shares of XYZ toward the $17.50 strike.

If the stock goes through $17.50, the people who are long calls at that strike will start selling stock against the calls. Again, that pressures the stock back down toward the $17.50 strike. In issues with large open interest, this battle plays out on both the put side and the call side. While it's hard to make predictions, it's a good idea to be aware of which strikes have the largest open interest, because they can act as magnets.

Pinning Candidates

Here are some stocks that are trading near strikes that still have a high level of September option open interest. Valero ( VLO) is currently trading around $111 per share, and the $110 strike has 12,000 calls and 9,000 put contracts still open. After getting slammed Wednesday Best Buy ( BBY) might find some support at the $45 level, as there are over 11,000 puts still open but just 3,000 call contracts at the $45 strike. Dell ( DELL) always seems to pop as a pin candidate; this time it's the $35 strike, which has 31,000 calls and 29,000 puts open, that could keep the stock hovering at the current price.
Steven Smith writes regularly for TheStreet.com. In keeping with TSC's editorial policy, he doesn't own or short individual stocks. He also doesn't invest in hedge funds or other private investment partnerships. He was a seatholding member of the Chicago Board of Trade (CBOT) and the Chicago Board Options Exchange (CBOE) from May 1989 to August 1995. During that six-year period, he traded multiple markets for his own personal account and acted as an executing broker for third-party accounts. He appreciates your feedback; click here to send him an email.