$controller

Creates a channel to manually set a single axis position or angle on a bone. Not recommended for use, mostly these exist since Half-Life 1 used them.

Warning:You can't assign more than 6 controllers to a single bone.

Syntax

$controller <number> "<bone_name>" [L]<axis>[R] <start_angle> <end_angle>

where

<number>

The number of the controller (you may need multiple controllers). 0-3 for custom controller, 4 or "mouth" for jaw (GoldSrc only).

<bone_name>

the name of the bone to control.

<axis>

The axis can be either X, Y, or Z. You need to specify a new controller for each axis of rotation.

[L]

The prefix L on the axis specifies that this controller loops back around to the start when it ends (see example below).

[R]

The suffix R specifies the angles are in  radians instead of degrees (see example below).

<start_angle>

the offset start angle of the rotation in degrees (or radians).

<end_angle>

the offset end angle of the rotation in degrees (or radians).

Example

$modelname "npcs/robot.mdl"
$cdmaterials "models/npcs/"
$model "Robot" "robot.smd"
$controller 4 "jaw" X 0 20
$controller 0 "tracker" LYR -1 1
$sequence "idle" "idle.smd"

Todo: confirm syntax of "tracker"