When new feathers first come through the skin after a moult they are sharp and bristly and difficult to grasp. These are called pin feathers. Plucking machines cannot remove them. As they grow longer they open out at the tip and look like small paintbrushes. Eventually the feathers lengthen and fluff out and look normal.
Loss of feathers (moulting) and replacement with new ones occurs at regular intervals. Textbooks will tell you when this is expected to occur, so that you can plan slaughter when there are no pin feathers present. However this is not an exact science.
Moulting is somewhat unpredictable, being influenced by breed, age, temperature, light levels and feed. Therefore you need to be able to recognize when your birds SHOULD NOT be sent for slaughter because it is impossible to do a good job of plucking.
Before loading birds for transport run your hand up the breast and if it feels bristly that bird should stay home. If the breast is smooth we can produce a clean carcass.