From Valve Developer Community
|l4d2=1 to show two inputs that are for entities available in L4D2 only.
- Move the brush to the end position (starting position + (move direction * move distance)).
- Move the brush to the starting position.
- Move the brush to a specific position, where 0 is the starting position and 1 is the starting position + (move direction * move distance).
- Set the speed and update immediately.