From slagcoin: http://www.slagcoin.com/joystick/restrictors.html
Neutral is the place where the shaft stands when not touched, ideally midway between each opposing direction.
The deadzone is the space between the switches where no directions get engaged (including neutral).
An engage happens when one or more switches are pressed. Engage distance refers to the distance/angle between neutral and the engagement of a given switch or set of switches. A shorter engage distance means a smaller deadzone and a larger engage distance means a larger deadzone. The engage distances are determined pretty much by how the joystick is manufactured with the placement of the switches and sizing of the actuator(s).
Engage zones are areas where one or more switches are engaged. These zones are determined by the engage distances, and by the size, shape, and placement of the restrictor.
The throw is the place where the joystick gets stopped in a particular direction. Throw distance is the maximum distance/angle the shaft can be moved in a particular direction. The edge surrounding the movable area of the joystick (all the throws) is composed by the restrictor. Restrictor gates specifically shorten and redistribute the throw distances.