A deal from an online Swiss Pairs with IMP scoring.
An instructional deal from a recent IMP pairs.
After a simple auction to game West led the diamond king (king for count) then the ace. East followed with the five and the four while South played the queen followed by the eight. West now continued with another diamond, declarer ruffed in dummy while throwing a heart from hand, drew trumps and claimed ten tricks.
What went wrong? West thought that declarer had QJ98 of diamonds and maybe Ax of hearts and not the ace of clubs. In this case a heart switch would be too late as declarer could get his hearts away on the established diamonds after drawing trumps. If West played a third diamond though East would ruff that and South could only get one discard on the diamonds. East should have played the nine of diamonds on the first round (the highest he could afford) and then the four on the second round of diamonds. West would have been able to read this more clearly and would have switched to a heart.
Surprisingly four spades made more often than not, mainly played from the North hand on a top heart lead.