There are several theories that suggest why double sequential defibrillation and/or vector change defibrillation may be effective.

Energy theory: increased energy delivery allows for defibrillation of a critical mass of myocardium terminating the refractory VF.
Setting up theory: the first shock reduces the threshold and results in a more successful second shock.
Multiple vector theory: change in pads (or additional pads) changes the energy vector defibrillating parts of myocardium not adequately defibrillated in standard position.