How a Shifter Works 

Here's how the automatic transmission shifter on your center console works to shift gears.
The gear shifter on your car is responsible for providing the driver an input method to changing the direction of the transmission in an automatic car. It is also a safety device preventing the shifter from being bumped into gear, ruining the transmission.
Two mechanisms are at play, to protect those around the vehicle and the transmission itself. The park - brake lockout secures the shifter in the P position until the key is in the ignition and the brake is depressed. The shift control computer, located underneath the shifter, will determine if these conditions are met and activate a solenoid to move a tab to allow the shifter to move.
The second mechanism at play is a detent rod that glides along a toothed profile. The rod must be pushed down (the button on the shifter depressed) in order for it to move into gear.
Manual transmissions do not have a park lockout or a toothed profile preventing certain gears from being selected, and hence must be used with caution.
This shifter was taken out of a 2001 Toyota Corolla with a 4 speed automatic transmission.