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 single-hook model is the special training needed to be able to really use the knives.
Once you are comfortable with the single-hook 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.