Looking into it…
http://www.darkfiber.com/pz/chapter1.html
Hypotheses of the hand gesture
Only two basic hypotheses regarding the hand gesture's meaning have been presented, both from this century and both based more on desires to decode this one single instance of the hand gesture rather than to provide an analytical discussion of the phenomenon of the gesture as a whole – a classic example of not being able to see the forest for the trees. The two theories are:
that the hand gesture is a secret sign indicating that the gentleman is a Marrano – a crypto-Jew who accepted Christian baptism in order to remain in Spain after the Catholic Kings' order of 1492 that all Jews leave Spain; and
that the gesture indicates a Loyolan/Jesuit spirituality that calls for the sinner to place his/her hand on their chest after committing a sin as a sign of moral pain. Both of these theories have been discussed at length in academic circles without, however, formal acceptance of either, though at least one academic allows that the hand "assumes the function of an important communication, a personal avowal."