What does a knock out bar for a mini lathe do?

The headstock of a lathe is pushed into a tapered hole in the head spindle. It gets stuck in there in normal use, and you cannot pull it out when you want to change it.

So . . . the spindle is hollow all the way through the lathe head. To get the headstock to lossen you insert a round knock-out bar into the front of the spindle until it touches the back of the headstock. A few taps with a mallet will jar the headstock loose. Then you remove the knock-out bar.