One of the major differences when learning to trade options as opposed to equity trading is the impact of time on the various trade vehicles. Remember that quoted option premiums reflect the sum of both intrinsic (if any) and extrinsic (time) value. Also remember that while very few things in trading are for certain, one certainty is that the time value of an option premium goes to zero at the closing bell on expiration Friday.
While this decay of time premium to a value of zero is reliable and inescapable in our world of option trading, it is important to recognize that the decay is not linear. It is during the final weeks of the option cycle that decay of the extrinsic premium begins inexorably to race ever faster to oblivion. In the vocabulary of the options trader, the rate of theta decay increases as expiration approaches. It is from this quickening of the pace that many examples of option trading vehicles gain their maximum profitability during this final week of their life.
Some of the most dramatic changes in behavior can be seen in the trading vehicle known as the butterfly. For those new to options, consideration of the butterfly represents the move from simple single legged strategy such as simply buying a put or a call to multi-legged strategies that include both buying and selling options in certain patterns.
To review briefly, a butterfly consists of a vertical debit spread and vertical credit spread sharing the central strike price constructed together in the same underlying in the same month. It may be built using either puts or calls and its directional bias derives from strike selection rather than the particular type of option used for construction. For a (long) butterfly, maximum profit is always achieved at expiration when the underlying closes at the short strike shared by the two vertical spreads.
The butterfly has the interesting functional characteristic that it responds sluggishly to price movement early in its life, for example in the first two weeks of a four week option cycle. However, as expiration approaches, the butterfly becomes increasingly sensitive to price movement as the time premium erodes and the beast becomes increasingly subject to delta as a result of increasing gamma. It is for this reason that many butterfly traders restrict their use to the more responsive part of the options cycle. For a butterfly, the greatest sensitivity to time (and, therefore, profit potential) is reaped in the final week of the life cycle of the butterfly, i.e. expiration week.
Senior Options Instructor