A Simple Example
North can make a grand slam in clubs. Say East leads a safe heart.
North wins the heart lead, cashes the top diamonds, and ruffs a diamond high.
North crosses with the ♣ J
, and ruffs another diamond high.
North then cashes the two hearts hearts, pitching spades from his hand.
Finally, North crosses with the club ace and draws East's trumps.
If East leads a club instead, then North uses the heart entry
to get back after the first diamond ruff.
Does the defense have a sacrifice?
A 7 ♥
sacrifice does not succeed - the defense takes three top hearts immediately
then attacks clubs. That kills the West hand, forcing him to ruff, and
never gets his spade tricks. East/West can score three trumps and a diamond,
but that's it.
But 7 ♠
is a fine sacrifice, always insured of five trump tricks, for -2000.