You are East. The bidding went:
West North East South
3C 3H 4C 4H
Your partner, West, led the CA-C2-C9(reverse count)-CT, and then switched to the S8-S2-SA-SJ. What do you lead to the third trick?
At the table East reverted to clubs, hoping West had started with five clubs only. South ruffed, knocked out the HA and had ten tricks. East-West made their three aces, but no more.
At trick 3, East should return a spade. When West plays the S5, East knows West began with a doubleton spade. On coming in with the HA, East can give West a spade ruff. The S8 could be a singleton (possible, but not likely) or top from a doubleton or from QT8.
South’s SJ could not be a singleton. That would mean West had led the S8 from QT85. Impossible. From that, West would lead the S5 or possibly the SQ. When East returns a spade at trick three, if West shows a doubleton, the rest is easy. If it transpires that West began with QT8, the spade return does no harm. East can still play for a second club trick when in with the HA after the spade position has been clarified.
From Ron Klinger in the 2018 Gold Coast Congress Welcome Bulletin.