As the shepherd snatches: The people of both Israel and Judah believed that, because they were chosen, God would save them and never let them die. The prophet’s words are ironic. Their rescue would be like a shepherd arriving too late to save the sheep, only able to pull two legs or a piece of an ear from the lion's mouth.
just the corner of a bed or the cushion of a couch: This points to how little would remain after the Assyrian attack on Samaria. It suggests that only small pieces of furniture would survive the destruction.
In the same way, only a few people in Samaria would escape. Their rescue would be as incomplete and fragile as the broken remains left behind.