Moving a Sprite using a behavior

I am using a behavior to move an object in TGB. It works great! but there are a couple of things I don't get:

On the moveMap.bindObj( ) the moveup, move down, etc, etc are bound with keys. However, when I look at the actual functions that move the object (moveUp, moveDown, etc), the code passes a %val parameter that in the function body it gets assigned to %this.right, %this.left, etc.

1st Question: Where did this %val come from? What does it do???

2nd Question:
Also, on the updateMovement function, the linear velocity assignment functions pass the speed parameter like this: (%this.right - %this.left) why is that?

I know thanks to this the movement is made smooth, vs exclusively assigning the linear velocity the the object, but I don't get how they work. I would relaly appreciate any explanation.

Code:

if (!isObject(MovementBehavior))
  	{
  		%template = new BehaviorTemplate(MovementBehavior);
  
  		%template.friendlyName = "Movement Behavior";
  		%template.behaviorType = "Movement Styles";
  		%template.description  = "Shooter style movement control";
  
      // this next field creation also effectively creates a variable with a string called upKey with a value of "Keyboard up"
	  	%template.addBehaviorField(upKey, "Key to bind to upward movement", keybind, "keyboard up");
  		%template.addBehaviorField(downKey, "Key to bind to downward movement", keybind, "keyboard down");
	  	%template.addBehaviorField(leftKey, "Key to bind to left movement", keybind, "keyboard left");
  		%template.addBehaviorField(rightKey, "Key to bind to right movement", keybind, "keyboard right");
  
	  	%template.addBehaviorField(verticalSpeed, "Speed when moving vertically", float, 20.0);
  		%template.addBehaviorField(horizontalSpeed, "Speed when moving horizontally", float, 20.0);
  	}

	function MovementBehavior::onBehaviorAdd(%this)
  	{
	  	if (!isObject(moveMap))
  		return;
  
	  	// bind our keys to the keyboard. here you obtain the value of the previously saved string, and assign a funciton to them
  		moveMap.bindObj(getWord(%this.upKey, 0), getWord(%this.upKey, 1), "moveUp", %this); 
	  	moveMap.bindObj(getWord(%this.downKey, 0), getWord(%this.downKey, 1), "moveDown", %this);
  		moveMap.bindObj(getWord(%this.leftKey, 0), getWord(%this.leftKey, 1), "moveLeft", %this);
	  	moveMap.bindObj(getWord(%this.rightKey, 0), getWord(%this.rightKey, 1), "moveRight", %this);
  
  		// set the default values to 0
	  	%this.up = 0;
  		%this.down = 0;
	  	%this.left = 0;
	  	%this.right = 0;
  	}
  
 	function MovementBehavior::onBehaviorRemove(%this)
  	{
  		if (!isObject(moveMap))
  		return;
  		%this.owner.disableUpdateCallback();
  
  		// remove the keybinds
  		moveMap.unbindObj(getWord(%this.upKey, 0), getWord(%this.upKey, 1), %this);
		moveMap.unbindObj(getWord(%this.downKey, 0), getWord(%this.downKey, 1), %this);
		moveMap.unbindObj(getWord(%this.leftKey, 0), getWord(%this.leftKey, 1), %this);
  		moveMap.unbindObj(getWord(%this.rightKey, 0), getWord(%this.rightKey, 1), %this);
  
  		%this.up = 0;
  		%this.down = 0;
  		%this.left = 0;
  		%this.right = 0;
  	}
  
  	function MovementBehavior::updateMovement(%this)
  	{
		// make the player move
  		%this.owner.setLinearVelocityX((%this.right - %this.left) * %this.horizontalSpeed);
  		%this.owner.setLinearVelocityY((%this.down - %this.up) * %this.verticalSpeed);
  	}
  
  	function MovementBehavior::moveUp(%this, %val)
  	{
  		%this.up = %val;
  		%this.updateMovement();
  	}
  
  	function MovementBehavior::moveDown(%this, %val)
  	{
  		%this.down = %val;
  		%this.updateMovement();
  	}
  
  	function MovementBehavior::moveLeft(%this, %val)
  	{
  		%this.left = %val;
  		%this.updateMovement();
  	}
  
  	function MovementBehavior::moveRight(%this, %val)
  	{
  		%this.right = %val;
  		%this.updateMovement();
  	}