I learned from my grandmother, it was an old worn out paring knife sharpened to almost an ice pick shape, but still with a cutting edge. She could stick a chicken running the blade the blade into the mouth and up into the brain at about a 17 degree angle, hang it upside down by its feet, watch until the feathers relaxed and run her hands from tail to head and 90% of the plucking was done with one or two passes, also works with turkeys. Then the head came off using the same knife with a little slice between the first vertebrae and the skull.
You see the same influence when you try to catch a chicken, it will shed feathers easily if scared - to leave you with a hand full of feathers and chicken gone. For some neurological reason this does not work when you cut their heads off first.
She let me try, I failed miserably several times and got to pluck the chickens, finally learned. Have not done it since 1959, but bet it still works.