Body velocity humanoid.MoveDirection.z not working?

I want the player to move foward and down at the same time when a player holds down "wasd" keys. although the player moves down slowly, the player's character does not move toward the position the player is facing. It doesn't move at all. Is there something wrong I'm doing?

local orientation = plr.Character.Humanoid.MoveDirection.z

mouse.KeyDown:Connect(function(key)
    key = key:lower()
    if key == "w" or key == "a" or key == "s" or key == "d" then
        while true do
            wait (.01)
            plr.Character.LowerTorso.BodyVelocity.Velocity = Vector3.new(0, -81,    orientation)
        end
    end
end)