A bid made in the full expectation that the contract will go down, but in the hope that the points lost will be fewer than those that the opponents would have scored if left to play in their own contract.
It is a near certainty that 4♥ will make. West may bid 4♠, not expecting to make it but in the belief that the penalty in 4♠ doubled will be cheaper than letting the opponents make 4♥. The vulnerability is often crucial in sacrifice decisions.
At duplicate, the best time to sacrifice is when only the opponents are vulnerable. In this example, you could afford to go three down doubled and still show a profit (losing 500 instead of 620). If, however, only your side is vulnerable, you need to get out for one down to show a profit (losing 200 instead of 420)
