From reading the rules, my understanding is that, when a player is in an impossible position, he must play as if trying to hit a ball which is 'on' if it was not for the obstructing balls(s). If he does so with sufficient strength (in the referee's opinion) to have reached the 'on' ball, were it not for the obstruction, the referee must then call only a foul. However if he judges that insufficient strength was used in playing the shot, he must then call both a foul and a miss.