In Example 1, S2 occurs at 270 ms. Not enough time has passed for the membrane to have recovered enough for a second stimulus to initiate another propagating action potential.
In Example 2, S2 occurs at 282 ms. Enough time has passed for tissue to the left of the stimulus to initate another propagating action potential, but tissue to the right has not repolarized sufficiently. Therefore, the wave that develops propagates unidirectionally.
In Example 3, S2 occurs at 287 ms. Enough time has passed for tissue on both sides of the stimulus to initiate another propagation action potential, resulting in a bidirectionally propagating wave.
The equations for this model (Beeler-Reuter or BR) are taken from Beeler GW and Reuter H, J Physiol 1977; 268: 177-210, and can be found in Appendix **.