When to use
When the market is either below A or above C and position is overpriced with month or so left.  Or when only a few weeks are left, market is near B, and you expect an imminent move in either direction.

Profit Characteristics
Maximum profit is credit for which spread is put on.  Occurs when market, at expiration, is below A or above C, thus making all options in-the-money or all options out-of-the-money.

Loss Characteristics
Maximum loss occurs if market is at B at expiration.  Amount of that loss is B - A - credit received when setting up positions.  Break-evens are A plus initial credit and C less initial credit. 

Decay Characteristics
Decay negligible until final month, during which distinctive pattern of butterfly forms.  Maximum loss acceleration is at B.

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