This is a ko. Assuming is a ko threat and black answers with then black cannot kill by playing under the stones as shown in the next two diagrams.
How about this ko threat at ? ~ Gregory
If Black replys to ko threat at then White takes ko and Black has to find the big ko threat to force White's response and then retake ko.
If Black ignores the ko threat and takes with , then .
If @ b then @ c, and if attacks on top at e or f, then at a captures the four Black stones with the double cut; and if connects on the bottom then at f gets the second eye.
If on top at d, e, or f, then still at c, Black connects on the bottom at D-1 or F-1 to protect against the double cut, and at b makes White's second eye. (In the case of at e white may fill to protect and threaten the cut at f making f and b miai for the second eye.)