When a bathtub gets clogged with hair and debris, you need to remove the twist down stopper from the bathtub drain to get the clog out. A twist down stopper has four pieces that hold it to the bathtub: the drain body, the lift stopper, the brass post and the knob on top of the post. The drain body screws directly into the bathtub flange, and you can disassemble the other parts from the drain body without removing the drain body from the tub.
Check the body of the drain stopper for a tiny "set" screw. Loosen the screw with a flathead screwdriver; the stopper should pull right out. If not, go to Step 2.
Pull up on the knob to set the stopper in the "open" position.
Hold the body of the stopper and turn it counterclockwise until it threads out of the drain body. If you can't get it to come out, go to Step 3.
Hold the body of the stopper in place while you unscrew the knob on top. Turn the knob counterclockwise until it threads off. When it comes off, you'll see a slot in the top of the brass post.
Slide the tip of the screwdriver into the slot and turn it counterclockwise until the rod comes free from the drain body. Pull the twist-down stopper up out of the drain body along with the post. If you need to remove the drain body, go to Step 6.
Insert a drain key into the drain body and turn it counterclockwise until it's removed from the tub flange.
Set the small pieces outside of the tub to keep them from going down the drain.