"Feather" is a rowing term that refers to holding the oars parallel to the water while they're above the water—and if a rower feathers too soon, they'll "catch a crab," or get the oar stuck. Alice doesn't understand this, so readers familiar with the terminology can laugh at her expense. Again, language can be expansive and mean many different things, which allows for all sorts of jokes depending on what a person does or doesn't know.