P. 39
A natural roll is the actual number rolled on a die, regardless of modifiers (including the halving of the result rolled on a d6 when rolling a d3). So
if something happens on a natural roll of 1 on a d6, even if you aim have skills etc if you roll a 1 this thing happens.
if something happens on a naturall roll of 1 on a d3 and you roll a 2, the result is 1, but this thing doesn't happen. If you had rolled a 1, it would have happened.