Playing another club looks safe but unnecessary, because if declarer has a third round club loser it is unlikely to disappear.

In fact if you do play another club, declarer will win A♣, play three rounds of diamonds and throw West in with the third club. West will then have to concede a heart trick or give a ruff and discard. Contract made.

To beat the contract East must switch to a red suit, but which one?

Pick the wrong one and declarer will make the contract with the same throw in as when a club is returned.

The answer lies with West’s play in the trump suit.

West should play 8♠ on the first round and 2 ♠ on the second, which is a SUIT PREFERENCE SIGNAL for the higher ranking side suit.

Had West wanted a diamond switch she plays 2 ♠ followed by 8♠