The theory is that you can push down the plunger and a "U"-shaped metal hook emerges. You grasp a sewing machine needle in the hook of the tool and release the plunger and the needle is now held securely in the tool and it will be easier to insert in the needle clamp of the sewing machine. Once the needle is installed, you push down the plunger again to release the needle. I could not order just one to examine, the minimum quantity I could order was three, so I ordered three.
When they arrived, I took one out of the package for a look-see. The third time I pushed the plunger, the metal hook broke off, the plunger fell out and the spring shot across the room.