means: All at once. It may refer to the swooping of a predatory bird, with the indication that there was a cessation of life in the prey because of the word 'fell' which is seldom used except in the context of (that tree was felled some time ago)
Sarah: "I am going to go buy a Wii, let's go to Best Buy!"
Angie: "I need to get some milk at the grocery store, why don't we go do both, so we can rescue two squirrels in one fell swoop."
Sarah: "Perfect!"
