By skipping a strike on one wing you change the economics: the wider wing costs less to buy, so the whole package can be opened for a credit. A credit broken wing has no loss at all on the narrow side — the worst case there is keeping the credit — while the wide side carries the width difference as risk.
This is the structure for a directional lean with a defined disaster. Traders use it to express a mildly bullish view where a sharp drop costs nothing, or the reverse. The catch is assignment risk on the short body strikes and the temptation to treat a credit entry as a free trade, which it is not.
Example: XYZ at $50. Buy the $50 put, sell two $47.50 puts, buy the $42.50 put for a $0.10 credit. Above $50 you keep $10. The risk lives between $47.50 and $42.50, capping out at $240 if XYZ collapses — and nothing at all if XYZ rallies.
Related: long-butterfly, iron-butterfly, jade-lizard, defined-risk