There are two very similar programs that are almost identical on processing 1.0.7 in the included examples section called Linear and SetupDraw, which show a horizontal line crawl up the window. This is Linear Code:

/**
 * Linear Motion. 
 * 
 * Changing a variable to create a moving line.  
 * When the line moves off the edge of the window, 
 * the variable is set to 0, which places the line
 * back at the bottom of the screen. 
 */
 
float a = 100;

void setup() 
{
  size(640, 200);
  stroke(255);
}

void draw() 
{
  background(51);
  a = a - 0.5;
  if (a < 0) { 
    a = height; 
  }
  line(0, a, width, a);  
} 

I don't understand why the line will keep getting redrawn continuously to create the basic motion as there is no loop called, just an if conditional test. Unless it is because of the word void in that it won't return a variable value and changes the variable value inside so will keep looping, is that right? That is a bit subtle I reckon and should be made more explicit in the code, code comments and even the language reference section Any thoughts?

When in doubt check the reference

Oh my mistake I overlooked that point.