South is in 3NT and West leads the SQ. Plan Play.
Clearly, in isolation the safety play to bring in five tricks in diamonds is to play a small diamond from dummy. So, let us say you win in dummy and play a small diamond. If, according to plan, (to cater to the 4-1) RHO wins the DQ. If diamonds are 4-1, he will attack dummy's heart entry while diamonds are not yet disentangled. He plays a heart. Now you have to try diamonds from the top and you go down whether the diamonds broke x QT9x or T9xx Q
Let us back up to the 4-1 breaks that you can reasonably handle in view of the entry attack. Do you see the farsighted play at trick one and two that is needed on the only layouts you can make?
In addition to 3-2 breaks which land the contract easily, you can only protect against 4-1 breaks with the SINGLETON QUEEN. Indeed, in this case, you preserve the spade and heart entries in dummy, winning the first trick in hand and DUCKING a diamond. No harm if diamonds are 3-2. The gain comes when you have the 4-1 break with the quingleton. The winner can only remove one major suit entry in dummy. You unblock the DJ winner, use the other major suit entry to enjoy 4 more diamonds and the contract.
Credits to Bridge Master, the learning program on Bridge Base . COM