A double finesse is a finesse that's taken against two opposing honor cards:
South is on lead and needs two spade tricks. A double finesse can be taken by leading a low spade to dummy's T.
If East shows up with either the J
South can take a second spade finesse upon regaining the lead.
This is superior to finessing the Q
whenever West has both the king and jack.
However, when faced with a 9-card holding, it may slightly better mathematically to finesse the queen:
In this layout, South is in hand and needs all 5 heart tricks. It's best to finesse the Q
and then play the A.
(Note that if South only needs 4 heart tricks, the correct line is to lead to the A