Quote from Sailing:
Tim,
Good Question:
Not knowing which way the market will move.... is why. The original Double Diagonal has a big 'dip' in the middle, approximately halfway between the two near month short strikes. The severity of the dip is determined by the width of the short strikes and VEGA.
As the position moves one way or the other, it's nice to 'bring up' the middle. This is accomplished by purchasing a Put Calendar (or diagonal) slightly out of the money. We use half the number of original contracts for the calendar.
We prefer the Put Calendar over the Call diagonal because it has a slightly smaller cost (because of the put skew) and because it has a tendency to respond to VEGA more quickly.
By adding the TENT pole (put calendar) you've now increased your profit window (break evens slightly) and given yourself an opportunity to profit nicely in the middle of the range. This is especially nice when the market moves up (over sold, puts are inexpensive, volatility is low) and pulls back for a couple days. It even creates a great scalping opportuinty within the time frame of expiration.
This certainly is not the 'holygrail' of Put Diagonal trading, but throw in a little haircut margin, a few tent poles... and relax. It's much different than trading spreads.
M~