There are two different types of gear selector inhibitor - mechanical and automatic.
Mechanical gear selector inhibitor
The gear selector can be moved forward and back freely between N and D. Other positions are locked with a latch that is released with the inhibitor button on the gear selector.
With the inhibitor button depressed the lever can be moved forwards or backwards between P, R, N and D.
Automatic gear selector inhibitor
The automatic gear selector inhibitor has special safety systems.
From park position - P
To select a gear position other than the P position, the brake pedal must be depressed and the ignition position must be II.
From neutral position - N
If the gear selector is in the N position and the car has been stationary for at least 3 seconds (irrespective of whether the engine is running) then the gear selector is locked.
To be able to move the gear selector from the N position to another gear position, the brake pedal must be depressed and the ignition position must be II.