In the case here and I suspect in all cases double dummy Gib has decided that ducking doesn't matter.
This all fine but Gib can make a mistake as it usually doesn't know all the cards.
I am thinking this may be part of the reason Gib used to not cash an ace on lead against 7NT.
Here my example contract is 2S by south. South led 6D, west TD, north JD Gib east has played 5♦. Gib knows the diamond situation so ducking doesn't hurt but it doesn't help either.
NOTE: This example is to show that Gib purposely delays taking tricks. The actual hand isn't that exciting.
My link