TGB Collision nodes? How do they work?

function playerClass::updateVertical(%this)
{
   %yVelocity = %this.getLinearVelocityY();
   
   %this.setLinearVelocityY(10);
   %collision = %this.castCollision(0.005);
   
   %normalX = getWord(%collision, 4);
   %normalY = getWord(%collision, 5);
   
   // no collision
   if (%collision $= "")
   {
      %this.airborne = true;
      %this.setConstantForceY(100);
      %this.setLinearVelocityY(%yVelocity);
      return;
   }
   
   // collides with wall to the left
   if (%normalX == 1 && %normalY == 0)
   {
      %this.againstLeftWall = true;
      %this.setLinearVelocityX(0);
      %this.setLinearVelocityY(%yVelocity);
      return;
   }
   
   // collides with wall to the right
   if (%normalX == -1 && %normalY == 0)
   {
      %this.againstRightWall = true;
      %this.setLinearVelocityX(0);
      %this.setLinearVelocityY(%yVelocity);
      return;
   }
   
   // on ground with no wall collisions
   if (%normalX == 0 && %normalY == -1)
   {
      %this.airborne = false;
      %this.againstLeftWall = false;
      %this.againstRightWall = false;
      %this.setConstantForceY(0);
      %this.setLinearVelocityY(%yVelocity);
      return;
   }
   
   // in air and hits platform with head
   if (%normalY == 1)
   {
      %this.airborne = true;
      %this.setLinearVelocityX(0);
      %this.setConstantForceY(100);
      %this.setLinearVelocityY(%yVelocity);
      return;
   }
   
   // in case another type of collision normal was detected
   error("another collison type" SPC %normalX SPC %normalY);
   %this.airborne = false;
   %this.againstLeftWall = false;
   %this.againstRightWall = false;
   %this.setLinearVelocityY(%yVelocity);
}