Movement speed is either controlled with the variables speed and direction, or hspeed and vspeed. I think hspeed/vspeed are used internally (horizontal and vertical speed, respectively) and setting direction + speed will have some trigonometry happen behind the scenes to influence them. There's also the variables friction, gravity, gravity_direction to influence moment but I find it easier to do it manually, especially gravity (since you usually want to limit how much momentum can be built up and the default gravity system won't do that).