The mandarin duck knives (also often called deerhorn knives) are a weapon special to baguazhang. They have four pointed tips and nine curved sharp edges. Practice of the mandarin duck knives teaches the player how to use long power to apply a short weapon. The snake model is the special training needed to be able to really use the knives.
Once you are comfortable with the snake changes you can play with the knives until they become natural and comfortable. The applications are all obvious, and the feeling is like a swallow flying freely. Do not train the knives by doing the changes carefully like a routine, play with them like you are fighting.
The knives should be large enough that when held end to end they protect the body from knee to head (squatting down). In this way they can almost be used as a staff. They are able to close the distance on an opponent very quickly, and can deal with a long weapon quite easily, if you handle them correctly. With all the edges and points, you need to be very clear about what, where, and how exactly you are cutting or stabbing when you practise.
You should hold them firmly, so that they become an extension of your arm. Also, holding strongly will prevent you from doing useless actions with your wrists, and make you apply your power through from the whole body.