What is the insight of Frank Starling law of the heart?
This is the short answer from wikipedia :) :


  As the heart fills with more blood than usual, the force of cardiac
  muscular contractions increases.[2] This is a result of an increase in
  the load experienced by each muscle fiber due to the extra blood load
  entering the heart. The stretching of the muscle fibers augments
  cardiac muscle contraction by increasing the calcium sensitivity of
  the myofibrils, [3] causing a greater number of actin-myosin
  cross-bridges to form within the muscle fibers. The force that any
  single cardiac muscle fiber generates is proportional to the initial
  sarcomere length (known as preload), and the stretch on the individual
  fibers is related to the end-diastolic volume of the left and right
  ventricles.
  
  In the human heart, maximal force is generated with an initial
  sarcomere length of 2.2 micrometers, a length which is rarely exceeded
  in the normal heart. Initial lengths larger or smaller than this
  optimal value will decrease the force the muscle can achieve. For
  larger sarcomere lengths, this is the result of less overlap of the
  thin and thick filaments; for smaller sarcomere lengths, the cause is
  the decreased sensitivity for calcium by the myofilaments.


in one sentence, it is a response to tension.