Basically, you just need to set a new keybind; once you activate the keybind, it checks some variable keybindToggled and if false, sets $mvMoveForward and $mvMoveBackward to 0.5; if false, to 1. If you need more clarification than this, ask; but hopefully you can figure it out from here.
Also, instead of using if statements, you could use a single switch statement as such:
//Keybind stuff up here.
switch(%isToggled)
{
case 0;
$mvMoveForward = 1;
%isToggled = 1;
return;
case 1;
$mvMoveForward = 0.5;
%isToggled = 0;
return;
}
Obviously, you'd need %isToggled to not be a bool for this.