Setup: sell 1 ATM call, buy 2 calls 5–10 points OTM, same expiration. Below the short call's strike, the structure expires at the credit (or break-even if zero cost). Above the long call's strike, the structure profits unbounded — two long calls vs one short. The danger zone is in between, where the short call has been blown through but the long calls haven't yet started printing.
Best deployed when expecting a binary catalyst with potential for an explosive upside move (FDA approval on biotech, takeover speculation, breakout from technical resistance). The zero-cost or near-zero-cost entry is what makes the structure attractive — you're risking the gap zone in the middle for unbounded upside if the move materializes.