East's Lighner double requested an "unusual" lead (not a heart or a trump); this double is generally based on a void suit. If West could have put his hand on a diamond, the contract would be down five!.
Alas, West reasonned that East's void could be in either minor. If it were in diamonds, the ruff might not be necessary, because West rated to win one or more diamond tricks. But if it were in clubs, the ruff might be crucial. Hence, West led the J. Oops.
South now can succeed, but it requires exact play. Can you make six spades after the J lead?