This animated video does a good job of showing all the components of a square hay baler and how they function.
1. First the pickup head picks the hay off the ground and moves it up into the bale chamber.
2. Next the plunger moves towards the hay, first cutting off the hay that didn’t make it completely into the bale chamber and then it compresses the hay against the existing hay in the chamber.
3. Finally when the hay in the bale chamber moves far enough it trips the knotter fingers and they present the twine to the knotter where it makes a knot and the bale is complete.