A door handle controls the door by the manipulation of a latch which connects the door with its frame.
Every time you push down on a door handle, you are applying force. A spindle is a square metal bar that passes through the door connecting the handles together and operating the latch. When you push a handle the spindle, usually 8mm in width, rotates.
This turning motion rotates a cylinder which is connected to the latch. As it rotates, the energy transmitted retracts the latch bolt allowing the door to open. In most cases, the spindle is spring-loaded which means the latch springs back into the closed position when the handle is horizontal to keep the door secure when it’s closed. The latch’s position is determined by the handle.
If the door handle doesn’t return to its normal horizontal position, it’s usually because the spring has broken. Replacing a spring is relatively easy but you will need a pair of special pliers.